Stock, the fastest sedan is the M5, but the C63 is faster fully tuned. The R35 Nissan GT-R is the fastest premium 2+2 regardless of whether it's tuned or not.
The IS-F can even be Race Modified if you didn't already know.
I know the C63 had more power than the M5 fully modified, but I prefer the M5 for racing - mines is fully modified but on Sport Softs and it still is less of a handful than the C63 on Racing Softs. But yeah, the C63 is probably the faster car if tuned right.
If that's the case, then the sedan you pick will come down to the track. If the track has lots of straight sections and not very many tight, technical bits, then the C63 will blow the M5 out of the water. (The C63 can be turbocharged and will have a significant power advantage, the M5 can't.) However, at a tight course like London or Tsukuba, the M5 should win because of how much better it handles. Test them both out and see which you prefer.