I always had the impression that lighter wheels (thus lighter unsprung weight) had to do mostly with keeping the wheels on the road (or off-road) surface. Heavy wheels have more momentum and when bounced up take longer for the suspension springs to bring them back down into contact with the road. Lighter wheels get back down on the road again quicker so as to propel you or steer you.

It also has an impact on the ride quality because when a heavy wheel is bounced up, it takes more force from the car's body to stop it and throw it back down again. Force that you can feel because it throws up the whole car.

Off-the-line acceleration improvement due to lighter wheels is mostly due only to the overall vehicle weight savings - not that significant.

Motorcycle design is a lot more involved with trying to minimize unsprung weight because it is a bigger proportion of the vehicle.

I really like the alloy wheels on the old 300D, but if it came with steel wheels I'd never replace them because they'd cost more than the car did.

Ken