Certainly interesting to hear FPS gamers advocating trackballs, do you not find that you can get precision or response, but not both?
I would have though you could achieve the speed of a mouse user by using high sensitivities, but surely that would reduce the accuracy or precision or any movement?
The alternative would be to have lower sensitivity and therefore have the precision but not have the same 'flicky' speed required for some FPS's
Out of curiosity, which FPS games do you play?