That level of weight reduction is a big win. Also, the weight/power ratio would be slightly higher than the M5, which makes sense from a cannibalization/product risk standpoint. If that's the case, performance should be pretty close to the current gen M3 CRT and GTS.
Looks like automobile magazine blog posts (that everyone poo-poo'd on including 'quasi-insiders') turned out to be spot on:
http://www.f30post.com/forums/showthread.php?t=749717