I found a file that benefits from prefilter, but only from "fast" and up. It is from an 448kbps AC3 track of a DVD. I'm omiting the normal and high modes as they behave in the same way as fast and extra in all the situations I tested.
prefilter-test.wav
TAK and diferent presets and prefilters strenghts
Mode Compression Speed
Turbo + off 40.15 % 45x
Turbo + low 40.20 % 35x
Turbo + medium 40.31 % 35x
Turbo + high 40.38 % 35x
fast + off 39.95 % 35x
fast + low 37.83 % 26x
fast + medium 37.83 % 25x
fast + high 37.83 % 25x
extra + off 39.77 % 12x
extra + low 37.33 % 10x
extra + medium 37.33 % 10x
extra + high 37.33 % 10x
If I re-encode to vorbis q4, the benefits of pre-filter grows a little for all presets, except turbo, where the compression hurt is even higher.
If I re-encode to vorbis q-2 (~32kbps) turbo starts to get benefits, but not much, and in an strange patern:
Mode Compression
Turbo + off 37.45 %
Turbo + low 37.28 %
Turbo + medium 37.31 %
Turbo + high 37.30 %
fast + off 37.12 %
fast + low 34.83 %
fast + medium 34.83 %
fast + high 34.83 %
extra + off 36.12 %
extra + low 34.15 %
extra + medium 34.15 %
extra + high 34.15 %
If I re-encode (it is geting a bit repetitive, isn't?) to Winamp's AACv2 32kbps, the figure is totaly diferent. Now Turbo gains a little with prefilter, and all other presets loose compression:
Mode Compression
Turbo + off 37.42 %
Turbo + low 37.29 %
Turbo + medium 37.30 %
Turbo + high 37.33 %
fast + off 36.83 %
fast + low 37.16 %
fast + medium 37.29 %
fast + high 37.32 %
extra + off 36.64 %
extra + low 36.88 %
extra + medium 37.00 %
extra + high 37.00 %
Now we have an extra mode that compress by defaut worse than the fast mode.
I can send it to you if you want. It is only 23 seconds long, and I'm keeping the vorbis and AAC compressed ones. May help you tune the prefilter, or even understand it better.