You really won't find any gain in changing around spark plugs and/or their heat ranges. If the heat range of the plug is correct and the plug is burning clean, that's as good as it gets. As mentioned, a modified engine might like a plug one heat range colder, and a motor that is lightly used might like one one range hotter. What matters is that the electrodes are not eroding, and that the plug is not fouling. This of course assumes that the mixture is correct to begin with. A good looking plug will have a light brown center insulator, and no evidence of high heat such as blueing and/or electrode erosion.

You can not change the burn characteristics of an engine, nor it's power capability by changing spark plugs given that is has the correct one to begin with. I've had great luck with Champion plugs through many years of hot rodding various engines, though often in two strokes the NGK plugs work well also. Just my experience, as I think really most any name brand is fine.

Good luck,
AV8TOR