you can get 65+ backs for pretty good money these days (relatively) if you don't mind shooting CCD and it's proper full frame too, also if you don't mind the huge 350Mb images it pops out...
On another forum there are some local folks who discuss Phase One IQ180 I believe (80MP version, I could be wrong the exact model) and compare it to GFX50s and GFX50s seems to be able to match it at very big print sizes.


