The car will never be fast unless you want to dump a good amount of money into it. Abandon that idea now.
The road handling is impeccable *until* you break traction. At that point, the open diff and drive by wire (in later models) kills any sort of control you may have to steer with your right foot. But here's the thing...the rear floor pan is so weak, that driving in a manner that would require you to have a LSD will eventually tear the mounting points out (you have checked these for damage, right?).
If you want to drive it like a hooligan, put reinforcement plates in the back and spend some money on a custom diff or full M3 setup. If you want a LSD for bad weather, i'd strongly weigh your options and get some good snow tires in the mean time.