I agree. It does however depend on (for the lack of a better word), "gearing" of the hydraulic pump. Some can spin faster due to their design/number of vanes in it.

But getting it to act like a centrifugal (correct me if im wrong) type super charger would not be possible as hydraulics always run at a specified pressure i.e, speed. The downfall with that is that when its used, pressure/speed drops until its fully working. That why you generally only find them in places like RAMS on trucks, tractors and implements which is where most of my experience from hydraulics comes from...

I suppose you could try to somehow plumb in a sort of pressure relief valve like a wastegate to control the amount of oil...?

However my main concern would be the fitting of an oil cooler because you would be looking at one the size of a small intercooler to be able to cool the oil... No matter what you do you will need a cooler. Hell, even my ride-on lawnmower which is hydro-static (hydraulic) has an oil cooler and its only used on a Kohler 25hp V-twin!