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Resonant, or "tuned" systems reach a point of diminishing returns at an exponential rate. The scale reaches a point which is effectively infinite rather abruptly. The most influential variable in this equation is torque. Incredible horsepower can be gained at a huge sacrifice in torque (by raising RPM.) Even by making that sacrifice, I think it would not be possible to move those numbers by a significant margin, especially considering the enormous compromise at other parts of the power band.
An interesting example of this compromise is a ram-jet engine. These engines are generally not capable of significantly greater output [in their 'power band'] than conventional turbine engines, yet the compromise they make is so severe that they won't even 'idle,' so-to-speak; that is, they only work at speeds several times the speed of sound. In contrast, a turbine engine is really nothing more than a supercharged ram-jet, which is capable of stellar performance over a wide, wide power band.
Another consideration is that a snowmobile typically uses a two-stroke engine, which has several advantages. First, the exhaust pulse is able to directly interact with the intake air. In a four-stroke, this interaction is effectively baffled by the valves. Second, because a two-stroke generates an exhaust pulse twice as often, you can tune to a higher frequency at lower RPMs. Third, two-stroke motors use the crankcase to generate intake pressure, further increasing power. Fourth, two-stroke motors are typically small displacement engines, capable of being spun very fast compared to most four-stroke motors which are designed more for torque. (This is not always true, but is generally the case.) Fifth, two-stroke motors are not expected to last for decades and hundreds of thousands of miles. ...because they don't. Even industrial diesel two-stroke engines don't last amazingly long, and their output per displacement ratio is fairly low, particularly when compared to a two-stroke gasoline engine.
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