Typically, valve springs are always an issue in running 4 strokes at high rpm. To hot rod one of these 4 stroke OBs, you'd need to install a higher performance cam, stiffer springs, and possibly lighter valve rain components. I suppose some cam manufacturer out there would develop and sell high performance cams for these things IF the demand was there.

Jeff

PS: Getting on plane with a 'hot cam' could be a problem here. It is for Go Fast boats with high speed propping.