Personally Alx, I see the C63 and C43 as very different cars, for people with different requirements.
Performance in a straight line is only part of the story and if I decided to modify a 43 I wouldn't be emulating a 63, because that's not what I'm after.
With respect, I'm not convinced it would be "a dot in the mirror" of a stock 63 anywhere other than the Autobahn.
The 43 is slightly lighter and (as Ian has said) will have less weight in the nose; with the right chassis set-up, tuning mods and the rear biased 4Matic it would tick more of my boxes for performance and handling across a range of road/track/conditions than the stock 63 can do out of the box.
Everyone's entitled to spend their money their way and drive whatever they want, so if you prefer the 63 I'm fine with that