i dont think disks offer any power advantage over rim brakes. your modulation would be better, but nothing locks like a magura. the problems with using a rear disk for trials are that it puts a greater stress on the seat stays and there's the possibillity of landing on the disk. you may also notice more flex in the bike as you brake. overall, if your rim brakes are working properly then upgrading to discs would be very costly for little/no improvement (you may even make your bike feels worse). if i were you i would spend the money on other parts of the bike where you'll really notice the difference.