Any implement made for the American market (and the small tractors we are talking about here) is designed for 540 rpm pto speed. (and unsafe to spin the implement much faster than that).
On your YM2000 to get 540 rpm pto output, shift the pto lever to the slowest position, that's 'standard 540'. 540 rpm output is attained at about 2200 rpm engine speed. If this mower doesn't bog down the engine, you might be able to run at lower engine speed and the next higher pto speed to again, have 540 rpm coming out of the pto shaft.
The higher speeds found in our ptos were intended for the implements they use in Japan, not applicable here.
As for ground speed I use my rotary mower in high range 3rd gear, higher or lower gear occasionally. You want a more uniform cut with a finish mower so this might be too fast to get a smooth cut on uneven ground.
Overrun clutch? The inertia of a spinning heavy implement can push you right into the side of a barn or over a dropoff before you can get the pto out of gear. Experiment and see if this lighter finish mower keeps pushing that hard after you put in the clutch and press the brakes. (Pushing in the clutch does not disconnect the mower from the drive train, there's still a direct mechanical connection through the gearbox from mower to the tractor's wheels until you shift the pto lever out of gear).
