Ahh, the quest for top speed. Every gear heads measurement of "Is this thing delivering what it claims to deliver"
All of thse machines power wise can easily hit the electronic limit of 67 mph as they could since the days of the GTX 185.
If your impeller and wear ring have any imperfections, they can take you down from there. The leading edges and the wear ring clearance being the most critical parts.
Next up is HP. Strangely from my experience, this is rarely the issue unless you have some serious compression loss or other factors such as old plugs, bad coils, etc.
When I bought my new to me speedster 150 255HP back in 2020. It failed to reach expectations beyond 7600ish rpm. It also had some surging. Like you, I was going to get that thing to 60 MPH.
First was a leak in the Intercooler. After that I had a slight stumble that turned into a full on mush over stutter and wild surging.
A new coil and bam. 8160 rpm and 61 mph. I could even whack out 63/64 mph in the cold start and end of the season at 8200 rpm. All of that due to a much larger intercooler from the 300.
In the first season, I ran the thing silly. Now I spend most of my time pulling the tube with screaming children, nowhere near 61 MPH.
But hey, If I want it, it is there...