Any advise on performance spark plugs please. Are Splitfires any good?

 

Nope (IMHO). Remember, with the splitfires, the power gains recorded were done on big power V8 motors. When you're pushing 700bhp, a 1% increase is 7bhp which looks like a good gain, but you'd do well to get within 1% on 2 dyno runs without any other changes i.e. that 7bhp gain may well be spurious.

None of my vehicles have anything other than NGK recommended plugs in them.

 

Yep, I have had best life from NGK over especially Champion