While the small oval (peanut port) heads are not the equal of the aftermarket options, I'm always puzzled when people say they are not good for more HP than the stock 330. Lots of street BBC running those turning out 450-500 HP. Here's an article where they got 525 HP with a mild cammed 496.
http://www.carcraft.com/techarticles...ads/index.html
Not a marine engine, but marine exhaust / cam could only take away maybe 75 HP - you're still at 450. The spings won't take a wild cam, but frankly a wild cam isn't necessary to get anothe 100 HP out of these heads.

Let the flames begin...