Unsprung rotational mass: one reduced LB of URM equals the effect of a reduction of 4 pounds of carried mass. If you choose wheels that are 5 lbs lighter that the OE wheels you will enjoy the benefits of an effective 80lb weight reduction of riding mass. But in real car terms it takes a greater reduction to produce a benifical difference that you can either feel in steering, turn-in, braking or acceleration. It my years with the of hobby tuning & sport driving Mini Coopers I tended to run 11lb forged racing wheels that were 16 lbs lighter than most OE wheels. My hunch is you don't want super light wheels on the HCH as it might produce a less than ideal ride.