The internals are strong for the C63 Black Series which still runs the factory engine internals but the CLK63 Black Series has a Weistec built motor setup with forged internal engine components. Those components are said to be good for over 1000 horsepower making them stronger than what the C63 AMG has from the factory.
Regardless, both cars also employ the 3.0 liter twin screw blower used in Weistec Stage III supercharged applications. The difference being one is making much more boost as the C63 AMG is said to be somewhere around 11 psi. What boost and other mods the CLK runs is anyone's guess as the owner tends to be difficult to deal with thinking his car is the fastest thing on planet Earth whose setup should be treated with the same secrecy as nuclear launch codes as if on the same level of importance. The CLK is likely somewhere in the 9XX rear wheel horsepower range while the C63 is somewhere in the upper 6XX wheel horsepower range.
So, on one hand we have similar base motors, although one built with aftermarket components, both running the same supercharger. On the other hand we have one car which has a Weistec built "Bulletproof" AMG 722.9 7G transmission while the other has the factory AMG 7-speed MCT. Weight is close from the factory but the CLK has been lightened quite a bit so that works in its favor. The CLK is tuned for race gas though while the C63 is tuned for pump gas.
So what is the result? Well, not hard to guess but nice to see the comparison between the two cars. Video below.