Splitfire's motto is they don't cost more until you use them. That's the truth as soon as you put em in you've spent more than you should have. They're garbage and a waste of money. I installed an MSD ignition and MSD said that stock plugs are the best way to go.
I will agree with everyone else on Spitfire spark plugs. They suck! I have tried using them in my '96 Seadoo XP 800 when I use to race, and they fouled regularly. Then I went ahead like a fool and installed them in my '91 5.0 Bronco, with no improvements. Now I just use the AC Rapid Fire plugs and they do well.