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The answer is no. It would take some work, and the C5 rear calipers are quite similar to the C4 parts, so there isn't really any point to making them work. Also, the C5 stuff uses a drum brake in the rotor hat for the parking brake. The late C4's didn't use this (I think the early cars did, but I don't think it lines up properly).
A friend of mine, Sidney004, is running J55's on the front with cooling ducts off the air dam, on a car he uses exclusively for the track, and he has had problems w/ the fronts overheating, but no issues with the rears. I would say ....don't wast the money converting the rears, but if you do alot of hard breaking, the fronts would be a good investment.
Sidney is alot harder on brakes than most people would ever dream to be......Last weekend at Thunderhill Raceway we got the "point by" by 2 Ferrari's and a bunch of GT3's. The only thing that passed this "old" C4 was 1 GT3, 1 friend in a C6, 1 vintage nascar testing for the next day.
If you want an expert on C4 brakes, contact Sidney004.
He passes cars w/ double the HP under braking.