I've got an X1140 as well. It's not a sport vehicle. Not by any stretch. But it's built WAY better than the Mule Pro I had before, and has much more control over tough terrain. It would be great if it had more power, but like others I'm looking for a work vehicle, and it serves that purpose really well.
If mine can hit 25mph, it's only on flat smooth terrain, perhaps a bit down hill.
Part of the problem sounds like you are not accustom to operating an HST. You said you slowed to a stop, then started going again and it was like it downshifted. YES, that's exactly how it works. If it's bogging down you need to back off the peddle, not push it harder. That lightens the HST load on the engine, and is just like downshifting. You also said how when you take your foot off the peddle it's like the hand brake was put on. YES, that's exactly how it works. If you back off the peddle, it will sslow to what ever speed the peddle now calls for, and it can slow down pretty abruptly. I gather it's very common for people new to HSTs to reject the RTVs, and it's really just because they have not been shown how to operate it.
As for adjustments, it's really important to verify that when the peddle is full down, the engine throttle is 100% open. Pushing the peddle works in tandem to increases engine RPM and adds HST load. Too much HST load with out enough RPM will bog down the engine and even stall it. Too little HST load and the engine will rev but not be fully loaded. You ideally want the revs a bit ahead of the HST load so you always have enough engine power.