A 3.38 LSD would be perfect, but you can only get a 3.46 LSD if you special order it, and that will cost you like $1700.
If you use your car as a daily driver, stay mild. A 3.91 LSD would
probably make you car a monster, but at 80mph you will be taching around
5000 rpms. The M3 tach is already high stock.