I dunno, Peter. Too many unknowns at present. I'm certainly very satisfied with my current camera. Yes, I only have one camera, which may surprise some. I guess if this hypothetical camera fitted in my housing then it might be nice alongside the D2X?
FF doesn't hold any particular interest for me. I rarely shoot ISO abouve 100. And I like more depth of field, not less. The main motivation would be for continued advertising sales. When I get paid big money for shots, it is good to reassure the client that they are taken with the top camera from my chosen brand. I miss the level playing field of film.
I would really miss the 10.5mm, which is a much better lens than the 16mm (which also does not focus close enough for my tele-fisheye technique). And I would also really miss the 15/16mm "cropped-fisheye" look on a APS-C camera. This is a very nice focal length for underwater photography - and one that is not easily available on FF. Also FF would prohibit my tele-fisheye technique, unless I bought a Sigma 15mm.
I would really like to use the 17-35mm as intended again, especially instead of the 12-24mm (which I have never owned, only borrowed). I have also never bought a 17-55mm DX, only borrowed.
Anyway, it is all vapour-ware so just idle speculation.
Alex