well, the common number people (on TV and fans alike) seem to get is 800 for the NASCAR cars, and I wouldnt doubt it, simply watch how quickly they accelerate
2nd. it's only a 300 pound weight difference - the NASCAR being 2700 something, with the C5R being 2500 or so... not even close to 1000, assuming those specs are correct
3rd, what would make you think the NASCAR cars have less downforce? if they have (at least) 80 more HP, they're 300 (or 1000) pounds heavier, and can go faster, unless a restrictor plate is fitted, which, about 32 times out of 38, it's not
Don't forget: NASCAR cars reach 198MPH on Charlotte, a 1.5 mile oval, what do you really think they'd hit on a 2.65 mile oval? (without the restrictor plate's)?
the record is 211, before restrictor plate were installed on the big tracks, and when they had less power than they do now. (again, without restrictor plate's)
P.S. the Chevrolet official top speed was 165mph, though we know it wasnt true, I have yet to see any evidence of 200, besides the horribly imprecise GT4