There are so many factors that could contribute to this. I would look at it from the outside in:
1.. What does you hull and keel look like? Are they still smooth and damage free? You would be surprised what affect a large gouge can have on top speed if it is in the wrong spot. Drag exponential and at top speeds can result in a loss of a few mph. Your ski at 230HP is pretty much drag limited as you can get around 67 MPH and that is not being limited by the ECU. 300HP skis are ECU speed limited as they can achieve higher speeds do to increased HP.
2.. Pump, Impeller, Wear Wing. If any of these are no longer in perfect condition, you may see a resulting drop in overall performance. I would inspect this for any rough edges on the impeller or wear ring.
3.. Temperature. These machines don't have oxygen sensors so they don't have the ability to trim A/F ratios based on post combustion readings. They basically have a fuel map based on MAP pressure and Throttle Position corrected by intake temperature. Your supercharger is geared to RPM so, it has a finite level of boost so there should be consistency there. Again, your engine adjusts based on intake temp, so the hotter your intake temp, the less dense the air is, = less fuel to maintain proper A/F ratios. The cooler the air, the more dense it is. The more dense it is the, the more fuel that can be added. The more fuel and air= HP. Try running your machine on a cool morning, then again in the hot afternoon. You might have different results. Also, your intercooler is what cools the intake and over time it can become less efficient do do a number of factors due to contamination etc. With the IC there is also heat soak when you shut it down after a hard run. Everything in the engine compartment is hot and absorbs that heat with no water flowing to cool it. Start it up and go for a top speed run and you will most likely have an intake temp higher than you had before when you were running it. My Speedster 260HP gained 200 more RPM with my new replacement intercooler. It is larger so will be more efficient, it also has the vent to prevent an air bubble forming in the top of the IC. This bubble can have an effect on performance due to many factors.
Again, there are even lots more factors but this might be some things to consider.