anything outside of the spec is going to cause you to lose power. however, given a choice, loose is better. if the valves are to tight, that means the valve is closing slower or not at all, which will produce less/no compression in that cylinder. If they are too loose, the valve will open slower, but at least it does close at some point. Valves should be adjusted within a few races of a rebuilt engine, and then every 6 or so races after that. For peak performance, you should also synchronize your carbs every time you make a valve adjustment.