Just thinking out loud here, but how cool would it be to swap in the S/C V8 from the Mercedes McLaren SLR?! It's the same size as the V8 in the E/S/SL/CL55 as far as I know, just beefed up.

On the other hand, they almost never come up for sale. Still, it'd be so awesome to shoehorn that engine into a Crossfire. 626 bhp ftw!