The LS2 ('05 + '06) GTO's are capable of being plenty fast and given equal drivers (almost never the case!) should beat a stock 335. Higher altitude obviously makes a big difference. I do frequent and am actually a member (only 11 post) over at the GTO forum:
Very entertaining "Kill" stories from these guys and some 335 races. We do get some respect over there but no GTO owner has admitted defeat at the hands of a 335. Guess that's why they call it Kill Stories and not "Killed".