2007 250 Pro XS- Completely lacking last 1/3 of throttle

[Bassh_nter]

I have been chasing a ghost desperately trying to discover what the issue with my 2007 Pro XS the past few months. The story-I have a 2007 Triton 21HP last fall I winterized the motor like normal (Placing oil in each cylinder, changing the gear lube and running Star tron through the fuel system). This spring I went to take the boat for the first run of the year and everything was normal until I went to open the motor all the way to do a hot lap. It was here that I first encountered the issue that has given me headaches all spring. The boat will hardly even touch 60 MPH @ 5500 RPM at WOT. Everything is normal all the way up to the last 1/3 of throttle. So in other words I can run at 60MPH with 2/3 throttle which is pretty normal but then when I push the hotfoot the rest of the way its like nothing is there, on some occasions I have actually witnessed a 1 MPH decrease in speed. Watching the merc monitor the GPH fuel consumption will continue to increase the last 1/3 to about 24 GPH even though no additional performance is noticed. I am running a 26P Fury and before the boat was put away for the winter achieving 70-75 MPH wasn't an issue. On a side note I have had the boat inspected by several marinas trying to diagnose the problem and all they were able to find was a leaking O ring at one of the fuel rails which they replaced. They saw no error codes in the system and tried running a new compressor with new plugs. I am truly stumped, any suggestions would be greatly appreciated!

[EuropeanAM]

Engine serial number?