I don't think BMW wants to potentially impact sales by allowing a comparison between an M5C and M5CS on a nice, clear day, hah. There's already several folks on dragy's leaderboard that have run high 10.7's to low 10.8's @127-128 in a stock M5C w/ 93 octane. As well this dude (
) on YT that recently ran a 10.77 @ 128 at the track in a bone stock '20 M5C w/ steel brakes and stock 789 wheels. Think that means same day, same track, we'd probably see the CS run a tick or two faster than that, so straight-line performance is a tad quicker but otherwise about the same. The reduced sound deadening probably adds to the effect of the CS "feeling" faster than it really is.
Where it sounds like the CS would make up ground is at a real track event, the weight loss and dialed in suspension feel has everyone that's pushed both cars making it sound like it's much more communicative on the track, which would result in more confidence and quicker lap times for sure.
If you want the more track capable car + the exclusivity, the CS is 100% it, just don't expect it to be much quicker in a straight line than an M5C (or for it to have center console storage, still don't get that decision BMW)