THAT said, I do agree - as well as the standard M5 vs E63S - that the Mercs have the better top end (as they usually do). So on two other dashboard to dashboard comparos - including one that was slightly biased (stating "See why Mercedes is better" lol) - have it nearly identical to 250 KM/hr.
In fact the M5C is leading the Merc by about a KM/hr at 1:55 to 160 KM/hr and 1:59 to 200 KM/hr. At 250 KM/hr, the MB is ahead by 2 KM/hr. When the GT63 hits 300 - the M5C is at 295 - which I believe is about right.