I have no idea why I said 345 hp in my previous post. Sorry about that. The STI is 497hp and 1219kg. It's max torque is 465ft-lb/5400rpm. I think it's really the difference in torque since the max hp of the M3 is 547hp at 1342kg weight.
Well I guess its the torque...but it would be both torque and horsepower if your running my setup. I don't ever use this M3 with more than 450hp...