Well if it is the "regular" SLS AMG GT with 583hp, it will be a close race with a slight edge to the SLS with its lighter weight. In the Motor Trend test it trapped 124.5mph. I would think that since the non-CP M6 trapped 123.1mph...the M6 CP would give it a run for its money at any speed. Ultimately a race like this would come down to the driver in the real world...but the M6 CP would have less margin for error.
http://www.motortrend.com/roadtests/...gt_first_test/