Basically following what Rapid Robert stated ,
I have seen very slight gains with a race type plug that uses a cut back electrode. With a regular plug and indexing it,I could not see a gain on my dyno with a combination I could get repeatable results on within .5 % .
As stated,sometimes you need to index a plug with a domed piston but we like to leave that to the Chebby guys with their heads with big open chambers. I would rather have a wedge design with a flat top or dish and good quench. Throw a race plug in and go.
Keith