I have a 4.10 in mine LOVE IT! ..top speed is still good if you can do 6,500 rpm.
but if your car is an "E" then i wouldn't go higher then 3.73
And about the Spider pin thing.. yeah i blew the old diff that way.. but the 325ix diff seems to be much stronger. Either way bmw's arn't made to rip gears, So i would avoid shifting very hard
The major difference between my 318is and an e30 M3 is that the noises come from the engine and not the passengers