Mister Hatt wrote: setting it as high as possible (make sure your machine has enough memory though) may improve the performance of the adaptive quantizer, especially on darker scenes.
Nooooope. Back when Dark_Shikari was on #darkhold, he explained that the max for slower settings is 60 because with animation and generally content with highly static/flat visuals higher mbtrees would make the video look worse
because it'd fuckup the bitrate allocation. Live Actions don't have that issue since there tends to be enough motion and detail and thus ends up fine, but with anime he was talking about potentially introducing some sort of rc-lookahead-min or whatever, which never ended up being done anyway.
Then again, mbtree's internal workings might have changed since then, so in that case it would be better to increase it.