I'm running a 3.64 LSD with my 6-spd (and 7k rpm red line) and love it. 70 mph is just under 3k rpm and even 85 is only ~ 3500 rpm, doesn't bother me or the wife at all. I'll admit that I never would have gone so short had I not had a unique opportunity to borrow a 3.64 for over a month prior to upgrading my own. After a road trip from Dallas to Austin, I knew I was completely comfortable with it. I'd definitely never run that ratio with a 5-spd though. I'd probably not run such a short ratio if I had a really loud exhaust either.
04 IR 330i ZHP | ESS TS2+ SC | MagnaFlow "headers" | KMS 3.64 LSD | AST 4100 550/650| APEX ARC-8 18x9 | H&R front sway bar | Vorshlag camber plates, RTAB limiters | TMS subframe & swaybar reinforcements | UUC SSK & DSSR | UUC TM & TME | BW motor mounts | Dinan strut & shock tower bars | vaio 4-bolt oil pump