Your engine is basically the same as the HP 500 on the bottom end so it is a good candidate for upgrades to the top. It is not difficult or terribly expensive to build your engine up by 100+ horsepower. Heads (or head work) valve train, cam, exhaust and carburetor will go a long way in the direction you want to head. As far as the silent choice - the answer is "It depends" mostly on your cam choice, a big cam will probably leave you with a reversion problem if you keep the silent choice. The brand of headers will affect this as well but someone else will have to help out here as I am not too clear on which ones work well and which to avoid. BTW, what do you have for a boat and where are you? Just curious.