Kind of related to the thread, I guess. Looking at the entry list to this years NASA 25 Hours of Thunderhill, Black Swan has a Merc SLS GT3 and GMG has 2 of the R8s entered for this year's race on Dec 7-8. I assume these are their world challenge cars. The other really interesting entry is the Audi TTRS that was their last year, I believe it is one of only a few factory build racing TTRS's. It's front wheel drive I believe, and was really quick last year before breaking after 5-6 hours.
www.nasa25hour.com If you live near northern cali, this is a really cool event with great access and lots of pro drivers who make the trip.