will putting a spacer, washer, shim, etc.. between the spring and spring guide produce more power or hender the performance? if so, what type of material should the spacer be made of...brass, bronze, stainless, plastic, or 1010 steel?

Shimming the guide end will increase the pre-load and fps slightly. If you add it at the piston end you add weight to the piston which should raise fps more and maintain the fit of the spring on the guide reducing any "twang". I think brass or bronze would have a better weight to size ratio. I have read "Naval" brass is what you should use.

Happy Canuck