Is there a quick change wheel offset set up that allows for rear wheel swaps between street wheel widths and to accommodate 15 inch wide Avons for the track?
Wheel spacers are not used, the different "offset" of the wheels is what allows the interchange of widths. Using a spacer is not recommended, the correct and safest way is the offset.
On a Superformance you can interchange the rear wheels from the 10.5" rear up to a 14" width by the wheel offset. The factory only supplies a 10.5 rear but we can get wider BRM style wheels or have the standard wheels widened.
What Frank has posted will work. I am not aware of anyone with a Superformance using that system, but no reason it will not work as the wheel and spacer are being held on by the wingnut and the pins are only transferring the driving/braking forces. The limiting factor is the length of the treaded hub so that you have full engagement of the nut on the threads. Wih a wheel of different offset that is not an issue as the wheel center is the same thickness (generally) and you have full engagement of the spinner. The "offset" of a wider wheel should be by the relationship of the center hub to the outer rim edge, not due to a thicker center hub.