I think that was the point of moving it over to Mercedes-AMG under the GT nameplate, is that it will be engineered to be more hardcore than the CLS63S which was really just an E63S in a prettier body. The target market that was buying the CLS wouldn't want any less comfort, so they split the CLS into 2 separate segments under 2 separate nameplates. All said and done it should drive somewhere between a GT-S and E63S than exactly like an E63S. It's a hell of a lot heavier and completely different platform than the GT-S, so yea, it will never drive like a GT-S. Of course, we'll see when it actually gets driven and tested.
MB toyed with this idea before by stretching the SLS chassis to make a 4 door SLS, but that idea was tossed due to costs. From a performance standpoint, that would've been the better path for someone that wants a 4 door 4 passenger GT-S.
The hilarious looking 4 door SLS patent:
Attachment 81386