might consider a different size/pitch prop for more torque under load and to compensate for the altitude? there are plenty of threads on this if you do a search.. good luck!
Do you think you need more to get outta the hole or on the top end? If you went to the stump puller and can't get outta the hole the alt is killing you. If you need more in the top end you went the wrong direction with the 1234/5 prop.
I think Mikeski did some HP increase things on his boat. New exhaust manifolds and maybe something else. He's on the board. Search him and send him an email.