Ya I don't have any experience boating at elevation so I can't say how much that is affecting your issue. I can say that regardless of your altitude your prop is holding you back. With your setup at sea level I feel like a 15.5x15 is a must but add your elevation on to it you might have to go to an even lower pitch #. You might reach out to ACME or OJ and talk with them as they should be able to recommend the best prop for your setup.