It seems shimming is to ensure proper gap between impeller and wear ring in order to maximize efficiency of pump. I haven't read about it creating an overheat condition. I'd continue to look for blockage in cooling lines. Engine and exhaust should have drain cocks. Also Check valve, if any, coming off pump to make sure nothing is in there blocking flow. Could also be something jammed in water feed port on pump....
Let us know what you find.
