You can compare FLAC compression with results of other lossless audio codecs which use arithmetic coding, like Monkey's Audio or optimfrog
(though the latter likely has a better predictor too).
Based on squeezechart wav stats:
Code:
SAC 21324487+65919345+20771857+22759276+25340833+52216710 = 208332508
OFR 21394272+65905363+20880497+23179339+25262790+52433236 = 209055497
MAC 21773012+66797828+21499324+23246616+25823788+52855336 = 211995904
FLAC 22369503+69547135+22708163+24927481+26791804+54394889 = 220738975
(1-211995904/220738975)*100 = 3.96%
(1-208332508/220738975)*100 = 5.62%
You can expect about 4% compression improvement over FLAC by using better entropy coding, and another 1.5% by using too much computing time.
In any case, residuals for testing should be easy to acquire since flac is open-source.