In my experience I have found that just by lowering the head you gain more power especially in lower rpm range, idle quality can suffer, the engine also ran cooler. I've ran my deck height really low before and didn't have idle problems but every engine is different. I don't run any big tracks so I am not sure how it effects top end performance but would assume that a higher deck height would be faster for top rpm speed on bigger tracks. With the copper shims for me I'd lose .001" head clearance for every .004-.005" in the shim(s) after the head was tightened. The guys with a lot more on-road experience than me should know how it effects the top end speed better than I do.