Well, there wouldn't be much point in releasing a new version that produces the same output, would there?
Ideally, you'd expect lossy encoding improvements to produce smaller files (lower bitrates) for a given sound quality level. I've run some tests encoding a variety of music with 3.98.4 and 3.99.5 and found mostly that 3.99.5 tends produce higher bitrates at -V0 and -V1 (sometimes a lot higher), while at the lower levels it tends to produce lower bitrates than 3.98.4. The biggest difference tends to be at -V0, and as you go down each level, the differences get smaller. Still, at any given level, depending on the music being encoded, it could go either way.
Edit: Here are my test results. Looks like in all cases -V0 produced a higher bitrate output, while -V2 was mostly lower, although not dramatically so in most cases.
Average Bitrate in kbps
Aerosmith - Sweet Emotion.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 249 219 198 166 152 139 126 106 97
3.99.5 (RareWares 32-bit) 264 223 189 168 149 129 112 99 83
Allman Brothers - Melissa.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 247 226 207 181 167 153 137 117 101
3.99.5 (RareWares 32-bit) 260 227 199 184 164 142 123 111 90
Bill Evans - Polka Dots And Moonbeams.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 227 205 188 162 149 135 119 100 84
3.99.5 (RareWares 32-bit) 243 205 177 164 144 124 105 96 80
Black Keys - Set You Free.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 261 232 202 162 148 135 123 103 94
3.99.5 (RareWares 32-bit) 258 219 185 165 146 128 115 98 79
Bob Marley - Redemption Song.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 215 183 166 152 139 121 107 86 73
3.99.5 (RareWares 32-bit) 228 197 167 151 128 103 86 78 72
Count Basie - Lester Leaps In.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 242 224 208 194 178 153 139 115 101
3.99.5 (RareWares 32-bit) 258 229 197 179 156 130 112 102 93
Cowboy Junkies - Mining For Gold.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 241 206 185 155 141 127 116 97 82
3.99.5 (RareWares 32-bit) 249 207 175 159 139 119 102 93 77
Del McCoury - Rambling Boy.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 242 212 192 176 161 144 129 107 94
3.99.5 (RareWares 32-bit) 266 226 193 177 155 130 111 102 89
Dizzy Gillespie - Manteca.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 246 221 201 180 167 152 137 116 101
3.99.5 (RareWares 32-bit) 263 224 196 181 162 139 119 108 94
Ella Fitzgerald - Anything Goes.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 167 144 131 116 107 96 87 72 63
3.99.5 (RareWares 32-bit) 187 159 137 123 109 92 79 71 60
George Baker Selection - Little Green Bag.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 256 233 213 184 170 157 141 121 107
3.99.5 (RareWares 32-bit) 268 235 205 190 171 150 133 120 98
George Jones - She Thinks I Still Care.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 231 198 178 161 148 134 121 105 93
3.99.5 (RareWares 32-bit) 264 212 180 166 147 127 110 102 87
Jack Johnson - Better Together.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 242 215 194 166 152 137 123 105 93
3.99.5 (RareWares 32-bit) 263 227 193 172 152 130 113 103 87
Jimmy Cliff - Ska All Over The World.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 227 207 191 179 166 151 135 114 100
3.99.5 (RareWares 32-bit) 249 218 191 179 160 133 113 105 93
John Coltrane - Giant Steps.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 269 227 203 184 171 156 140 126 113
3.99.5 (RareWares 32-bit) 292 238 201 187 167 145 126 121 102
Johnny Winter - Mojo Boogie.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 256 224 200 171 156 142 131 106 100
3.99.5 (RareWares 32-bit) 284 232 193 170 150 130 114 98 77
Kanye West - Gold Digger.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 220 190 172 147 137 125 115 95 87
3.99.5 (RareWares 32-bit) 249 213 178 159 143 125 109 96 77
Katia Lebeque & the Cleveland Orchestra - Rhapsody in Blue.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 230 208 190 174 158 130 118 93 79
3.99.5 (RareWares 32-bit) 247 210 178 159 135 108 91 83 82
Leo Kottke - Jesus Maria.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 212 191 176 162 148 133 118 99 86
3.99.5 (RareWares 32-bit) 236 204 178 165 144 118 98 94 86
Lotus - Sea Of Tranquility.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 203 176 160 148 135 113 104 84 73
3.99.5 (RareWares 32-bit) 229 192 160 142 124 102 88 79 69
Marcos Valle - Para Os Filhos De Abraao.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 239 207 186 161 148 134 121 102 88
3.99.5 (RareWares 32-bit) 261 221 186 167 148 128 113 101 81
Nirvana - Smells Like Teen Spirit.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 266 242 215 167 152 138 124 103 92
3.99.5 (RareWares 32-bit) 259 224 191 169 148 127 113 96 79
Ottmar Liebert - August Moon.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 253 231 214 198 183 167 150 132 114
3.99.5 (RareWares 32-bit) 299 248 218 202 181 157 134 129 107
Peter Tosh - Get Up, Stand Up.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 257 238 220 199 185 164 150 128 112
3.99.5 (RareWares 32-bit) 277 247 217 201 178 151 131 124 107
Pointer Sisters - Fire.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 242 213 193 165 151 136 123 105 92
3.99.5 (RareWares 32-bit) 261 226 192 169 148 126 110 101 89
Psychedelic Furs - Love My Way.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 244 208 185 162 147 133 121 102 96
3.99.5 (RareWares 32-bit) 262 220 182 164 145 125 110 98 81
Rolling Stones - Street Fighting Man.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 248 228 212 200 185 169 153 125 113
3.99.5 (RareWares 32-bit) 259 232 205 192 171 148 119 112 90
Shirley Bassey - Moonraker.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 241 216 195 173 158 142 127 105 89
3.99.5 (RareWares 32-bit) 261 225 194 177 157 135 118 103 80
Smiths - Please, Please, Please, Let Me Get What I Want.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 254 240 225 186 171 155 138 107 95
3.99.5 (RareWares 32-bit) 271 246 217 195 173 150 132 105 78
Steely Dan - Black Friday.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 244 219 200 171 155 139 124 105 92
3.99.5 (RareWares 32-bit) 266 231 196 174 152 128 113 102 85
Stevie Ray Vaughan - The House Is Rockin'.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 258 242 217 167 153 139 125 105 95
3.99.5 (RareWares 32-bit) 261 230 200 170 150 130 115 100 83
Sublime - Badfish.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 252 222 202 177 162 147 132 114 101
3.99.5 (RareWares 32-bit) 276 237 202 182 161 139 122 113 94
Thelonious Monk - Ruby, My Dear.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 207 177 161 149 137 121 110 87 72
3.99.5 (RareWares 32-bit) 226 196 166 150 132 100 80 75 80
Tomita - Introduction And Dance Of The Firebird.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 224 204 188 174 160 135 119 92 76
3.99.5 (RareWares 32-bit) 237 209 182 164 139 107 88 80 78
U2 - Bullet The Blue Sky.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 241 219 201 165 151 138 122 102 88
3.99.5 (RareWares 32-bit) 255 222 190 168 148 127 113 99 79
Van Morrison - Moondance.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 236 210 192 173 159 145 130 111 95
3.99.5 (RareWares 32-bit) 255 218 189 174 155 133 114 105 87
Will Ackerman - Abide The Winter.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 228 208 192 177 161 135 119 93 76
3.99.5 (RareWares 32-bit) 244 216 186 167 142 108 88 82 78
Yngwie Malmsteen - Leviathan.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 252 213 187 166 151 137 124 103 98
3.99.5 (RareWares 32-bit) 268 222 185 166 146 126 111 96 81
Yo-Yo Ma - Boccherini - Cello Concerto No. 7 in G major, G480 I. Allegro.wav
-V0 -V1 -V2 -V3 -V4 -V5 -V6 -V7 -V8
3.98.4 (RareWares 32-bit) 224 204 188 173 157 132 118 95 81
3.99.5 (RareWares 32-bit) 239 209 180 164 141 114 95 88 82