C4 calipers (both HD and standard) has 1.5" pistons, total caliper piston surface area of 3.53 sq in.
C5 calipers has 1.75" pistons, total caliper piston surface area of 4.81 sq in.

C4 rotors weigh 20.5 lbs, and is 1.11" inch thick.
C5 rotors weigh 18.9 lbs, and is 1.25" inch thick,
said to run cooler.
http://www.superchevyperformance.com/v/misc/c4c5rot.html

C5 has a larger brake pad surface area.
http://www.superchevyperformance.com/v/misc/c4c5pads.jpg

I do not know the performance difference between the J55 and the C5, since I went from the standard brake to the C5.
But when i did the change, there was a huge difference.

I thought it was one of the better bang for the bucks mod.