2002 Utopia 205 running slow... Help!

I am new to this forum and I am needing some advice. I am not a mechanic at all either. My 2002 Utopia 205 won't run above 40 mph. When I bought the boat 2 summers ago it would run about 55mph. The boat was like brand new and ran great. I decided not to trailer the boat last year and left it in the water all summer (not salt water). This is when I started noticing the drop in top speed. About every 2 weeks I took it out it would lose about 5 mph. I then took it into the dealer and they replaced the plugs which helped some. Then by the end of the summer it was only topping out at around 30mph. I took it back to the dealer and they told me there was corrosion in the impeller sleeve causing jetpump cavitation, so they replaced the jet pump wear ring. Now I just took the boat out for the first time since it has been fixed and it only tops out at 40mph again and rpms are at 6000. The boat sounds like it is running ok it just doesn't have the top end speed. There is some moss or crud along the bottom of the boat from it being the water for the summer. Would this cause about a 10-15 mph loss or is it something else? Any advice would be appreciated?