Looks like BMW took a crack at 'em and just fell short
http://www.bmwblog.com/2008/09/10/2009-cadillac-cts-v-vs-2008-bmw-m5/
I love the bias of the author...
Cliff notes... CTSV vs M5... each driven by factory driver respectively (for GM, their test driver who set the record at the 'ring with the CTSV, for BMW, factory driver who is on their ALMS team).
CTSV turns better time... excuse excuse excuse, blah blah blah... translates to BMW rips GM a new one.
Gotta love bench racing.