Performance-wise, this is my guess:

stock C63 > stock SRT-6
modded C63 > modded SRT-6 (talkin' bolt-ons here)
stock C63 =/< modded SRT-6

The fastest modded C63s have run 11.4, 11.5 and trap in the low 120s - these are all motor, no F/I or nitrous. Mind you, this is also basically after one season of running modded HW/SW, learning launch techniques, etc... wouldn't be surprised to see them run 11.2 @ 124 (or thereabouts) in ideal conditions this fall.

As for mixing up a CLK63BS ($140k MSRP widebody coupe with lots of CF, forged wheels, etc.) and a C63 ($60k MSRP entry-level AMG sedan) - well, that would kinda be like mixing up a CTS-V and a ZR1... JMHO.