Thanks
@Sergei1. I went to the dealer to pick her up today. I spoke both with the service advisor and the technician. When I asked the tech if there was any way to test the HST to verify it's not what's caused the external pump to fail, he said he didn't know of any way. My concern is that if I take the tractor home, and they close out the service ticket, I'm plumb out of powertrain warranty.
The tech said he'd submit a ticket to the factory to find out how to verify the health of the HST - which could take a week to nail down. In the meantime, the service advisor told me I could take it home, and she'd leave the ticket open until we hear back. If they provide an objective test, then either I will perform it, or return the tractor to them for them to do so. If it fails the test, it'll end up with a replaced HST (advisor said around $9k) under warranty.
Now that I have her back, I can say that the FEL does operate faster - though still not as snappy as the neighbor's much smaller Kubota.
As for forward speed, I was able to better test variables. When I have it it middle gear, RPM around 2100 and pulling box blade up a moderately steep driveway, the RPM will really bog down and forward speed really drops down. I do not recall it fall off so dramatically in the past on my driveway, but since it's the engine that is dropping RPM, and not just slowing forward speed (without RPM drop), I would assume that means it's unrelated to the HST?
It basically feels like the engine is not as strong as it used to be. It loads down faster than I recall. Of course, that's a pretty subjective observation, so I wouldn't bet any real money on my memory.