Tested by some amateur driver obviously..in the hands of a professional driver the gt-r and the ruf can both lap in less than 8 minutes, even got videoproof of a 7:59 stock gt-r lap. And seeing a 996 turbo can lap in 7:56 i suppose the ruf should easily lap in less than 8 minutes. And oh, the caterham's R500 superlight has been clocked at 7:55.
A good-ish laptime list here with sources included: http://forums.finalgear.com/showthread.php?t=14757
Still can't really be compared as almost every laptime's been done by a different driver, the ruf CTR2 is for example marked as 8:15 because there hasn't been proof of a better lap yet..it doesn't mean it couldn't do better.