Correct me if I'm wrong, but M mode just makes the throttle more responsive. Meaning, if you're in M mode and you push the accelerator halfway, the throttle mapping makes it more like 75%. I would say if you push the accelerator 75% of WOT, it's like WOT in non-M mode.
I played around with M mode a lot during break in as I don't recall it stating anywhere not to use M mode. However, I did wait til I hit exactly 1200 miles before I tested out the launch control.