I have the X-T1, which is brilliant. I use this camera over all the others I have. I find that with the 400mm it gives me a reasonably chance to get most things, if using the converter then I use the tripod and release.
In good light autofocus is as good as any mirrorless I've used. Not as quick as my Nikon DSLR in any other light conditions, but I use my Fuji as a lighter carrying set-up along with the scope, I find that I can carry this lot around and not get too knackered. It's super at close-up photography, especially plants, butterflies and bugs. Birds are good if you can get within range, but that's the same with any other camera.
In terms of ISO then there is a difference and limit on what you can do. But results, that's in the eye of the beholder, I'm fussy, what's acceptable to most wouldn't be for me. I'm lucky that I have software which is not available to others, so I can fiddle to some degree on the final look. But there's a limit to what you can photograph in dull light and be happy
The light needs to be good for this camera, but saying that I would be disappointed with the results others get with high ISOs using their DSLR.
The X-T2 has a bigger image file which means you are extracting more detail from the file, I find that tempting, but not enough changes from the X-T1 to warrant changing.
I would go to LCE and have a practice, I'm sure they'll let you. That way you can judge whether this camera is for you.
Both those are solvable with some sugru and alot more batteries in my pocket 
