It was clear that the bitrate difference between -q 2 and -q 3 would increase, but it's more or less negligible.
Looking at the rate of change of bitrate, I'm more inclined to leave -snr preset values as per v0.9.8c as it gives a more even spread of bitrate.
Practically speaking I prefer the way it's done with 0.9.8c a little bit too.
I've re-visited the -spf function for higher frequencies at longer FFT lengths.
was: spreading_function_string : string[precalc_analyses*(spread_zones+2)-1]='22222-22223-22224-12235-12246-12357';
now: spreading_function_string : string[precalc_analyses*(spread_zones+2)-1]='22222-22223-22224-12234-12245-12356';
this gives the following for my 53 problem sample set:
|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
| lossyWAV | -q 0 | -q 1 | -q 2 | -q 3 | -q 4 | -q 5 | -q 6 | -q 7 | -q 8 | -q 9 | -q 10 |
|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
| beta v0.9.8c |327kbps|364kbps|406kbps|445kbps|468kbps|500kbps|533kbps|564kbps|595kbps|624kbps|653kbps|
|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
| beta v0.9.8d2 |328kbps|365kbps|407kbps|446kbps|470kbps|501kbps|534kbps|565kbps|596kbps|626kbps|654kbps|
|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
10 Album Test Set:|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
| lossyWAV | -q 0 | -q 1 | -q 2 | -q 3 | -q 4 | -q 5 | -q 6 | -q 7 | -q 8 | -q 9 | -q 10 |
|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
| beta v0.9.8d2 |298kbps|???kbps|???kbps|???kbps|???kbps|463kbps|???kbps|???kbps|???kbps|???kbps|639kbps|
|---------------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|-------|
Attached is a detailed analysis of bitrate vs quality preset for each sample in my 53 problem sample set.