I have raced with the fanblades with the OEM 235/50-18 on all four corners and got a best of 13.34. Next time out I had the heavier, taller and wider Cobra 340SC wheels wearing Nitto 555 245/45 up front and 295/45 out back and my best dropped to 13.2s.
As five oh brian said, rotational mass reduction is good, but in the case of the stock wheels wearing the original size tires...I don't believe that weight difference will make much difference at all. The difference will be in the traction limits of each tire. In that case, I've seen the 17" Pirelli out perform the 18" BFG! Given that you could exactly duplicate the driving with both sets of tires, I'd guess the difference would be thousandths (.00X), maybe hundredths (.0XX)