Ive got a USA ski boat with a V8 5.7 litre Chevy engine. Had it for 10 years. For first 5 years I changed the NGK standard plugs every year. Had another unrelated issue with engine and engineer fixed it and then he recommended fitting Iridium plugs. They cost significantly more! Any way I fitted Iridium and got 400 more RPM , extra 5 mph and they have lasted 5 years so far!
