25 hp mows just fine on what little flat ground I've got, even on some of the more gentle upslopes. I use H1 for those areas. What's the biggest pain, is shifting both the 3 speed transmission AND transfer case, back and forth between H1 and L3 for the more severe uphill runs.
The tractor is very heavy for it's size, and needs a lot of that 25 up just to get ITSELF up a hill in H1. By comparison, my 24hp 4 speed Yanmar mowed the entire place - uphill and down - in H2. I attribute most of that to the Yanmar being 800 pounds lighter, and having more appropriately spaced transmission gearing.
Another consideration for a heavier tractor on hillsides, is that a 5' medium duty rotary cutter on the back end of the Jinma makes for some pretty light steering - uphill.
//greg//