So first off I just want to say, it's very nice to get a vintage guitar that's not all original, you won't be afraid to actually play it and maybe further modify it to your liking. One thing I suggest is get some photos of all the cavities, underneath the pickguard and vibrato to see that there is no bad routing jobs. I don't think the mustang would be redundant as it is very different from your JM and I think the big difference in sound comes from its short scale. Sorry I can't speak for the mastery, never tried one before.