Brenspeed (decent tuner, mostly parts pusher) dyno'd a customers car with some heavy 20 inch cast wheels vs the factory 18's.
Factory wheels weigh 49lb and ignoring 20 mm difference in tread width the aftermarket wheel weighed 61. So 24lb total difference netted 366whp vs 349whp. Almost a 5% reduction in power at the wheels! (11-14 5.0s dyno 365-370 whp stock through the mt)