In order for there to be a noticeable image quality difference, you have to double the diagonal. So you'd be going from 43mm to 85mm, give or take.
I dunno, the 645z (diagonal 55mm?) looks to have better high ISO capability than the A7s, which is about the cleanest full frame sensor at present. It won't be worth the massive price difference to most people, but it is noticeable.
I agree all this sounds unlikely though, anyway.
Current equipment: 5Ds, 5D mark III, 50D, 24-105L, MP-E, 70-200 f/2.8L II, 100L macro, 500L IS II; 1.4xIII + 2x III extenders; 600EX-RT.
Former equipment includes: 300D; EOS-M, EF-M 18-55, Samyang 14mm f/2.8, EF 35 f/2 IS, 70-200L f/4 non-IS, 85L II, Sigma 180 macro, 200L 2.8, 400L 5.6