Have been chasing this problem for a few years. kept losing performance and boat was going slower and slower all the time.. I could actually run at idle and only lose 2 mph from full throttle..

Finally tried everything I could think of and decided to replace the Carburetor!! Voila! Like I got a new motor!! Top speed is great again!!!

Also completely replaced the fuel system from the tank to the carb including a new fuel pump.
The in line filter / separator got replaced too. I don't think I was venting that as often as I should.
I was getting some random stalls and venting has solved that mystery.

I'm in Maine so there is little support up here so I easter egged a lot. Finally hit the right combination to make me smile again.