I don't know how heavy a NASCAR car is compared to an IRL car, but in an IRL car I think 60-80 pounds could make quite a difference over the whole race. (A few years ago, one of Dale Earnhardt's cars was on display at the Seattle hydroplane race, and I was surprised how small the car looked; maybe the weight differential between Indy cars and NASCAR isn't as much as I would think.)
I can see 80 pounds making a difference, you're starting to get into the serious fractions of a hundred then, but 50 or less, doubt it. And even at 80 pounds the difference is going to be slight enough to be easily overshadowed by driver skill.