I haven't upgraded MyBB nor XThreads for a very long time because I've been ill. Now I want to upgrade XThreads v1.47 to the latest version. I also want to upgrade PHP5 to PHP7.
I'm running MyBB v1.6.10 and unless required I'd prefer not to upgrade it, because I've made core modifications, and as I've had brain surgery I hardly remember a thing about anything that has happened a bit ago so I don't have a clue about what I may have changed years ago.
I don't think you should find major issues upgrading xThreads as long as you are not a newbie into doing so. IIRC each update has been as simple as just "upload, run the upgrader, done".
As for php, I don't know if it works fine with PHP7 really but I'd recommend to figure out all of your edits and upgrade to 1.8 anyways as 1.8 closure may be close too.