I looked at one of those diesel Rangers, and noticed it had some variant of the 3T72 engine found in many Yanmar and John Deere tractors, including my YM1401D and YM186D. They are rated at 14 and 15 PTO horsepower respectively, and something like 17 and 18 engine horsepower. The version in your Polaris is a bit larger in displacement, as I recall, but it's not much: Something like 25ccs sounds familiar.
I would speculate that the engine is about as "cranked up" as it can be for reliable service, especially with the Rangers having a reputation for running hot when modified anyway. The intermittent service engines seem to get most of the additional power from running at higher RPM. The Ranger engine spins to 3600 while the tractor versions are somewhere around 2700.
I'm certain that there is power to be gained with a turbo and adjusting the fueling, but you're the guy who will have to decide if it's worth it. Why not just buy the high output gasoline version? The 900cc version of the RZR is rated by the state of California at 89 odd horsepower! The 800cc Ranger, even the heavy Crew, is still producing 40 horsepower in stock form.
I don't see the point, I guess, in trying to tune up the diesel to similar performance figures. If longevity, durability, fuel economy, fuel commonality and low-speed grunt is what you need, then buy the diesel version of the Ranger. I don't expect the diesel will be outworking the others, though, except in the fuel economy figures and longevity, but the durability will be negatively impacted if you're tuning it to run with the gasoline motors.
My experience with Polaris engines has been excellent, and I'm a fan, but trying to boost the power of the diesel version seems like trying to make ones Porsche into a Baja bug. Sure, they're quite similar in layout, and even design, but.....why? Just get the one that's already designed for what you want to do. You're wanting to boost the power by 50% over what it already puts out. How long would you expect a tuner Duramax to live at 600 hp? Especially with stock everything else. I just don't see it. I hate to be a negative Nancy though. If you try it, let us know how it works!
There are some pretty wild builds of these types of small diesels (Yanmars and Kubotas) amongst the garden tractor pulling crowd; they may be able to offer guidance as far as specifics go. I don't have a link, unfortunately, but some Google searching and poking around will reveal them, I'm sure.
