I know that I have said this before but I just have to say it again -
the Quaife differential is one of the best upgrades that you can put in
your M3 (or other cars too). Yes, it is a bit expensive (I paid about
$2,400 with a 3.46 differential) but I would go with this rather then
those 18" wheels. This morning, it was yucky (foggy and wet) here in the
NY area and I decided to play with my newly added power (more on that in
a few days) and the traction control off. There were a few cases where I
accelerated and the car wiggled a little but still went straight when I
am sure that I would have been spinning in circles with a normal
limited-slip. It is just amazing how it gets the power to the ground.
Check out http://www.quaifeamerica.com/differentials/diffs.htm for more
information or speak to Brett Anderson at Koala Motorsport
http://www.koalamotorsport.com/ - that's where I bought mine.
P.S. I have nothing to gain by recommending this - I am just a satisfied