Shimming won't restore the clearance between the wear ring and impeller on the diameter. Most likely you have damage to the impeller or something that is disturbing the flow into and through the impeller. Climb under and take a look in the inlet of the jet to view the leading edges of the impeller. As stated before, any dings in in the entry of the impeller will cause cavitation that will continue through the pump. Same thing with dings on the leading edges of the bowl vanes. All will cause excessive RPM and lost thrust.
If you feel ambitious enough to pull the pump apart and the dings are not to bad, you can sharpen them out with a file. I wouldn't suggest going hog wild with a grinder unless you are willing to sacrifice your 1st part to the flow gods. A good clean-up will do wonders for your performance.
All that can be done for no cost except for some of your time and gaskets. American Turbine website has documentation on Jacuzzi pumps that you can download. Take a look and if you don't feel comfortable or you are not feeling mechanically inclined, you might consider getting help.
Good luck,
Joe
