I don't believe the splitfires make a motor any faster or more HP. I do believe, however, that if ignition may be an issue wether it's weak spark or poor fuel mixture they help. It may be they index themselves better or have a better exposed spark kernel.

 

If you want the spark kernel exposed with the price of a NGK, learn how to side gap a plug. Just take a die grinder and cut the ground electrode off even with the center electrode. The best of both worlds.