I don't have a belly mower, but can answer part of your questions:
How well do you like your LS XJ2025H
Very much. Mine is for sale now because I want a larger machine overall - especially when it comes to loader height for maintaining shooting berms. But for the small size, it is VERY impressive what this little tractor can do. Compared to other models this size and HP, you really get a lot for your $. Some minor annoyances like key position and twitchy loader joystick, but overall many more pros than cons.
What is the max speed in low range?
About 5mph forward, 3mph reverse. A little more than 2x that speed for high range.
Can you mow with MMM in high range going up hill?
I don't have so can't say for sure, but I can't with a 5' bush hog. Never tried, and probably depends on the hill. But low is all I can tolerate and 5mph is fast enough for me. The smaller wheels - esp front tires, short wheel base, and overall light machine (even with loader on and rears filled), unless you are on very smooth ground, it just beats you up too bad to go any faster. So even if I could bush hog in high, I wouldn't choose to do that due to the rough ride it would cause.
I would add, I think either a bush hog or finish mower on the 3pt is a better way to go with this machine unless it's primary purpose in life is finish mowing a lawn type area. Don't know the exact spec but I think ground clearance is around 9" - so that would be greatly reduced with a MMM. For me, I'd be constantly on/off with it and would prefer not to have that hassle - even though it is a pretty sturdy drive-over and looks like a nice MMM. Belly mower is going to be around low to mid $3k price tag - that goes pretty far into a separate ZTR for a much more efficient mower depending on what you're doing.