The difference is internal rotating mass and the C6 has a lot. Big heavy drums take power to accelerate. Little C4 stuff spins up fast, spins up easier when rollerized. If you set up the roller drivetrain in a C4, update the drums to 5~6 plates with bleed holes, update the oiling paths and use a 6 pinion planetary, updated input shaft and welded front drum you can really beat hell out of one. At something like 500 lt/lbs for sure you'd probably want to look inside after every season or so, depending on how many runs. I have a rollerized C4 that's been behind 300, 500 and currently about 460 HP motors and I haven't looked at the guts in maybe 8 years. Clicks off gears like a clock.
If you reduce the C6 internal rotating weight in some way, then you get power back. Just like in the driveline - a reduction in rotating mass is stated to be about 5 times more effective in freeing power than removing static weight. Hence the carbon fiber driveshafts, lightened gears, drilled axles, super light rims, etc.