I did the half and half thing with my last boat. It had 850hp ran just fine. Filled it up with just straight 87. Broke a roller rocker due to spark knock. Did half and half several times with no problem though. Don't use the marine grade octane booster. It does not have ethanol. Bring your own. I have never had it gum up plugs if you buy the right stuff. And I dont see how it would. It increases octane which burns hotter and cleaner. But buy the right stuff.