162, I don't think reeds are the problem. Reeds have been around for a long time, originally developed by McCullouch, the chain saw guys, back in the early 60's. On a 2 sroke, after combustion, as the piston goes down, the exhaust port first oppens, and most of the spent exhaust goes out this way. Unforunately, a few millimeters down the cylinder, either the carb piston port, or the tranfer ports open up and some of the exhaust gas gets forced back into the carb , which pulls gas up in the carb . The gas sits around for the intake cycle, maybe droplets form by then, and when the intake happens those droplets of gas, not a nice mist of air and gas, can cause problems, like low speed stumble, and other things. The reed valve is just a one way valve, that stops this, and make the engine a little more efficient.
Been awhile since I had a fan cooled, 88 340 Sport was the last one in our fleet, but I'm going to maybe blame some accountants, maybe, for the 550 problems. Heat in a 2 stroke is always a problem, especially as the power goes up. Air cooled 4 stoke diesels probably run a less than 3,000 rpm. 2 stroke sled engines turn 8,000+ rpm, with a power stroke every revolution. Run your air cooled diesel at 15,000-16,000 rpms, and see how they deal with heat extraction. I have not really looked hard at the 550, but you say it looks a lot like the 440/488 fans of the past. Back to my accountant problem, any chance the fan rotor , and housing, is the same size on the 550 as on the 488. My guess, and it's just a guess, is that someone at Polaris said, sure we can make more power with the 550, and keep the same engine case casting and fan, and not tool up ( meaning major $) for new engine parts. Looks like the real world may have proved that wrong. Maybe. The real problem has to be getting heat out of the engine, under all conditions. NicaSil cylinders will help, timing changes can be effective too. Was it the fan size, or something else? We may never know, but somebody in Medina knows, that we can be sure of. My $.02 worth.