i am driving a 4.10 fd in my e46:
http://www.m3racing.de/e46umbau.html#20
but the ratio really depends on the track(s) you are driving. if you calculated that a 3.91 would be best, give it a try. the e46 definitely has a too long stock fd of 3.62. compared to the e92 6mt which has nearly the same gear ratios 1-6 and has a 3.85 fd although having more power and torque output.
although for a track-e92 i would also opt for a 4.10 (or 3.62 for the dkg, that has 3.15 in stock version).