You will see gains with a Yamaha. Maybe even bigger gains than Merc. Yamaha tunes their engines on the conservative side so they don't have as many warranty issues. In general, on a boat running 55-60 it takes 7 hp to gain 1 mph. On a boat running 60-65 it takes 10 hp per mph. I would guess with the 225 you are getting around 52 to 54 mph. With a tune up to 250 I would imagine 56 to 57 mph and 300 tune it would run around 60. Hell, it may blow 60 out of the water, just remember, if you get all that extra speed, you are going to have to start reproping your boat. So a $800 remapped computer turns into a $800 computer and about $500 in gas and prop testing, then $1000 for a custom prop to squeeze every last ounce out of the boat. Haha, first world problems suck!