iridium? never heard that before. for boost some people say to stay away from iridium. as the smaller electrode can cause a flash point and create KR. the plugs most people use are the cheep autolite plugs. the copper core. stock for your gtp is A-606 most recomend one step colder A-605's with the copper i could say they should be either checked or replaced at 20k miles just to be safe you can prob go 30-35 also gap them at .055 if your stock or lightly mod'd