Coming from GPL and rally sims and LFS, I find the M3 pretty easy to handle when the tail comes out a bit. Not that I'm such a stud, but that this M3 is not a real beast in the grand scheme of race sims. The main problem is that a sim car doesn't give you nearly the feedback that you get in a real car.
Try easing off the throttle partially instead of just dropping it. Remember that when you go off the throttle, you will pick up some rear grip by reducing the torque on the rears, but you will also loose some grip because the weight transfers forward.
Anyways, jack those diff percentages up and a lot of that will go away. Same goes for the 320i in ETCC. The default diff settings are almost wide open, no good for a car with that much power. It handles a lot like the M3 if you move the power and coast percentages up to 40/20 or so.