Hi and thank you for your question.
For starters, the compression seems way too high. Is the engine modified?
Next, it sounds like you are describing reduced MPH.
That can be caused by one of two symptoms. Either reduced peak RPM's limiting top speed or normal peak RPM's but reduced thrust. In the first case, the engine just won't rev like normal which obviously reduce speed, the second case is similar to a clutch slipping in a car or motorcycle. The engine is running fine but the power is not being converted into thrust.
As far as the power valves go, if they get stuck in the down position, or a mechanical failure causes them to not open, that would create a symptom where it has normal bottom end throttle response and acceleration but go flat on top end, more than likely creating a reduction in peak RPM's due to reduced cylinder scavenging limiting horse power production. Yamaha's power valves have proven to be very reliable and not prone to mechanical failure. All power valves can be susceptible to getting stuck down due to excessive carbon deposits building up on them and causing a bind. Yamaha tries to combat this by incorporating a cleaning cycle where the valves open and close after the engine is shut off, with the intent of scraping carbon off of them.
It is fairly easy to check to see if the power valves are moving. First, start the engine and shut it off and watch the pulley that has the power valve cables on it. Does it cycle after the engine shuts off? If it does, the next thing I would do is remove the caps over the tops of the power valve linkage on the cylinder and watch the power valves to make sure they are actually being lifted by the shaft that is rotated by the servo motor via the cables.
The whole compression test results still bothers me. Unless you have a high compression head on the engine, it really is just too high. It makes me wonder if you the tester you were using is accurate or not. I am not trying to slam you here, but obviously something is wrong with your ski and we need to figure out what it is.
Since an engine takes spark, compression and fuel to run, we need to figure out which one is being lost and then determine why it is losing that to get you back up to speed.
Let me know about those things and we can move forward.