J55 & C5 brakes are both 13" rotors with dual piston calipers, but that is about where the similarities end - no parts are interchangable.
The C5 rotors are about twice as thick as the J55's. The C5 caliper is also a bit heavier duty and is a bolt in design unlike the J55's quick change pin style. Also that pad surface is bigger with the C5s.
basically the C5 rotor and therefore the caliper are not set as far back in the wheel as a C4. The inside part of the face of the sawblade wheels will hit the face of the caliper- where as the A-mold/C5/aftermarket wheels allows more clearance. Does that make sense?
You can see what I am talking about in this pic, notice how the Smaller C4 rotor hat sticks way out...which sets in farther back in the wheel.