I was shocked but they tested a Miata with light weight racing wheels vs some generic heavy wheels and found no difference in laptimes. They even went as far as to say that this was the 2nd time they visited this topic and again, no benifits to laptimes.
I have also felt a difference when switching between light wheels and heavy ones but when it comes down to it, I'm sure there may be some slight benifit but when you're dealing with a beastly 3500Lbs and (if you're turbo) over 300ft/lbs of torque, I bet the benifit is even smaller than a low torque miata.
I have a screen shot of the mag on my phone, I'll see if I can get it posted tonight..