Check KMS's site on this (http://www.koalamotorsport.com). Brett's got a good article on the diffs of diffs. I was doing the exact same calculations on my car last night.
My '90 535iA has a 3.91 LSD (cold weather package). At ~ 70mph, I run about 2900 RPM. If I put in a 4.10, I go up about 300 revs, to about 3200.
Not that bad, but I'd rather just keep the current gearing and save the money on gas.
Best, whit