well this is dangerous, insane and will mostly end with a broken xmod I LOVE IT.. just throwing out ideas but rather than addidng more wieght to your xmod (pcb, alothough not alot its stil added) you could wire the rockets to the motors brake function (not sure how) or seeing how this a soely drag racer, you could wire rockets to the seating control.. just make sure you lock the wheels up good.
and like many i wanna see a video of this, lol the look of your drag opponent's face should be priceless!