More power from 3 1/2 Tohatsu?

[jspringator]

My 2004 3.5 HP 2 stroke Tohatsu pushes my 12' Portaboat to 7-8 mph. My 6 HP 2 stroke Johnson (1975 model) pushes it to 11-12 mph. Both numbers are with 2 people one 200 lbs and the other 100 lbs, from a GPS. Are these numbers out of range? Is my 6 hp under performing? I get the impression that 12 mph is all the boat will do. For the weight difference, the 6 may stay in the garage more. Can I juice my 3.5 to get a little more speed? Prop change? I have the original plastic prop that came on when I ordered it from Internet Outboards. How about a 6 pitch aluminium? That Tohatsu is sure a lot lighter and easier to deal with.