Observations on my V20

Yesterday I decided to take my V20 out of the brine and to a fresh water river to blow some salt out of it. Well I had pumped my 60 gallon tank out due to the fact it has some water in it. I had added some water evoperator to the tank, so I just took a 12 gallon and 6 gallon portable tank. I filled both up. I put some Ring Free in each tank, a some mix of outboard oil, and about half a can of Seafoam. Started it up and I was on my way. I have a 1990 Yamaha outboard. I was running off the six gallon tank w/ an in-line filter with a sight glass by SeaChoice. I am spinning a 14 1/2 x 17 pitch ss prop. At 5000 RPMs (WOT) she was up to 40 mph and trimmed out all the way. She preformed great. Well, I am after a bit more speed and increased RPMs. I know the prop basics. But I am sure you guys have a tried and true method to get more speed out of these motors and boats. The boat is excellent on gas. Out of that 6 gallon tank at full throttle for 75% of the time I went 17 1/4 miles. ABout 3.8 MPG. Not real bad I thought. I check the compression yesterday on the warm block and she read, 110lbs to120lbs all the way. I am using regular BH8HS-10 NGK plugs what the motor calls for, any suggestions on better plugs? Give me some feedback on these situations guys..Thanks....Dirk and Al