one reason they dump so much fuel is to cool the pistons. i was read an faq on top fuel engines and i also remember that the blower actually uses up 700hp. and they still make 5000+hp.
the mileage a cup car gets really depends ont eh track they are at. at the shorter tracks they get less than at the longer tracks due to the gear they run.
Part of the reason is the fuel itself. Nitromethane is self-oxidizing to an extent(rocket fuel afterall). It carries molecular oxygen so less air is needed to make it burn. They run a/f ratios somewhere between 4:1 and 6:1 which kills mileage. Not only that, a cup car makes about 1/7 of the power a top fueler does.
Just for comparison's sake, diesels can run a/f ratios up in the hundreds to one.