i know its obvious, but with others like engine/brake/ps fluid you can go between serviceable limits without noticing much. i was only down maybe 1/5th of the way on the tranny fluid (i noticed a tiny leak), but added honda atf and it runs noticeably better. however, i dont know what fluid was in there before (looks like they used regular 10w30) so its possible just the addition of honda brand atf made the difference (honda atf is pink).
im sure this can be applied to manuals too as ive heard of people resorting to heavier weights, ie 20w50. just try honda as its only a few bucks per quart and some people even swear by it.