from my own experience during the day i had the m6 and m6 competiton for test drive i had a chance to race against an sls on the german Highways from 150kmh to 280kmh in the m6 (not competiton)
it was dead even. But this was an standard sls and not the sls gt
the Video below shows aswell an sls from 0-300 in about 35 secs..
http://www.youtube.com/watch?v=V2aI7KvS4pk
wonder if others had some real life experience with other outcomes, or about the same as i had