I've been running the M3 in ST-U class since purchasing it. My previous car was a shoe-in for the class and was competitive (Mitsubishi Evo). The M3 is not so much in stock form... but I knew that from the start anyway. So far compared to my usual benchmark of regulars, I am down about 3 seconds on a 40 second course - that is a pretty big margin to gain back.
The car's power is not an issue, for me it was putting the power down without going power sliding around corners. Also, the soft suspension and lack of feedback aren't very reassuring. But I am getting used to the car now and a lot smoother than before. The car does have great turn in response though! Incredible turn in response.
Here's a video of one of my laps where I was throttle happy and go spinning.
The front stock camber is woefully inadequate even with a performance alignment. The tires are rolling onto the sidewalls in front. I'll probably start there and pick up some stickier street tires as well.
Great thing about a Street Touring class is that you run street tires which keeps the budget down and it is pretty open to a lot of mods as long as you don't change internal engine parts.