550 motors are for 4WD SCT's, an X12L would rip a 2wd gearbox to bits... My 4.5T x12L had literally twice the power output of a 6.5T X12!
8.5T competition motor is as hot as you want to go really in a 2wd SCT, they are funny and very quick With 6.5's in but don't handle well and you have to weight them so much you lose benefit of faster motor.
I run an 8.5T X12 LRP motor, on tracks with long straights it gets some boost added on speedo
