Engines are expensive, would it really cost that much more to run premium? Premium will give you a wider margin of safety against detonation, which will quickly eat your pistons if it happens. I've seen my fair share of two-strokes with scored cylinders because of stale low quality fuel, probably wouldn't have happened if the fuel was premium, even if it was a bit stale, but low octane and stale simultaneously was enought to increase combustion temperature enough to consume the lubricating oil before it could lubricate the rings and cylinder walls. This is only an example, of course your ski isn't two stroke, and it has a detonation sensor that will detect and retard the ignition automatically, but there are always limits. Can you discern detonation yourself, and back off the throttle if it does happen? I think you cannot, and if you could it might be too late to avoid damage. Do you ride your ski hard or store it for long periods, stale fuel loses octane.
Check the specs closer, does your ski really have the same engine? Is the top speed and horsepower spec the same? If so, that's a good case to support using premium. If your engine is the detuned version, then perhaps your owners manual and dealer are correct.
