Right,
I've spoken to Prof Yaffle and my brain now aches. Basically what I put earlier in the thread is sound, there is no easy way of calculating an effective loss/gain as there's too many variables in wheels, you just have to understand the principles of polar moment of inertia and the gyroscopic effect.
The bigger a wheel is, i.e. the further the bulk of its mass is from centre, the bigger its resistance is to rotational acceleration/deceleration, also once spinning the gyroscopic effect is what makes something harder to turn the bigger it is. So, larger wheels have a direct impact on performance in terms of acceleration, deceleration and steering response... steering is slower to react, it saps more power from the engine and the brakes have to work harder. This where the similarities are drawn in terms of it being equivalent of adding/removing xxkilos.
Even if a 17" wheel is the same weight as an 18", it will still respond better to turn and accelerate/decelerate quicker. That's why they're deemed to be the best for pure performance.
Unsprung mass is pretty much as I described. The lower the mass, the less effect it has on the car's suspension making the ride smoother.
Hope that helps.
