I didnt have the chance to race a kleeman E55 with my M5..i've raced more than 10 E55 MKB's with 620 hp,the reason why we have too many MKB's is that the mercedes dealer offers the tuning and their is no warranty issues.From my experience,the MKB E55 stays with me all the way to 180 km/h,from 180 to 220 the M5 is 1 car length ahead,thats why i'm not surprised at all since the M6 is a rocket at high speeds

.I have to mention that i didnt race from complete stop(approx 60km/h).
The stock CLS55 or E55 should be a peice of cake for you if your a good driver (even at low speeds).I have raced a SL55 with MKB stage 1 which consists of ECU sofware and racing filters (530 hp/ 730 nm) more than a million times,simply no chance for the AMG

.
here is the race link,note that the SL55 always steals the start:
Shorter video: BMW M5 vs Mercedes SL55 AMG MKB