when you adjust the shims do it when your engine is at normal operating temperature, my valves ticked all the time even at idle, and as the rpms went up the ticking got faster. power loss? well the engine is weak especially at higher rpms. and if your valves are out of wack then your engine isnt giving the correct compression to the chambers thus power loss. all my parts are stock and cost me around like 300 (just for parts) i.e. low compression pistons with all the rings and stuff, 16 valves, the head i sent to a machine shop to rebuild
the shims were around 12 dollars or so EACH from a nissan dealership, or you can have them made from a machine shop for like 40 dollars a pop, (but those are prices around here maybe your local machine shop will be cheaper.)
the choice is yours rebuild or just swap for the lower mileage one...
what will you do