I thought the top of the rpm range at wot for that motor was 5500. Is that not accurate? If so, you could go down 1-2 in pitch to see if that improves top speed if that is what you are after. Also, different props produce different performance, and on an older boat, any build up no matter how slight on the pontoons can reduce performance mph.
A 22’ G-series SPS tritoon with a 150 should be able to go faster on a light load…solid high 30’s.
As for maximum HP, if an SPS hull tritoon, it should be a 200HP motor. It will say what your max HP motor can be on your capacity plate on the side rail fencing near the helm and throttle. Look there, and you will know what your boat is capable of handling.
However, repowering from a 150 to 200 can be a VERY cost prohibitive venture. It can sometimes involve not just changing the motor, but also various lines and connections (not always, but often). If seriously considering that route, your local dealership service department might be able to give you an estimate on costs to repowner to 200HP. You might be surprised how much it costs, and performance gains would likely not justify the cost.
I think you are better off trying to dial in your set up as is, and gaining a good 5-8 mph that way.
