I suppose the degrade in IQ (in particular, the resolving power of the 645 lenses) would not be too much as this is exactly what Pentax has been doing/forcing us to do exactly the same thing with more or less the same crop ratio for all those K-mount film lenses of us when they are put onto *any* Pentax K-mount DSLR that was ever produced up till now! >:-[
Btw, the 645D is cropped in 1.3X on the original 120 FF format. So, this 0.3X difference could actually cost a lot, which I am not going to pay for that difference for that much anyway!
Consider also that the pixel pitch sizes of my 5D3 and the 645D are close enough, I suppose my FA645 lenses should be good enough to out-resolve the sensor at pixel level. :-)
Of course, for the overall resolution of the whole picture frame, the 645D still wins, i.e., 40MP over 22/24MP of the current 135 FF digital bodies which have the similar pixel sizes. Actually, this is what the line pairs per picture height or width (LP/PH or LP/PW) figure is going to tell.
Sample photos would follow.. The IQ of those old FA645 lenses on 5D3 is really impressive!
Related:-
What is/are the Meaning(s) of a Cropped Digital System Now?
Comparison of Sensor Specs - 645D Against Others
This is What the Limited Lenses Should be Put Onto!
DIY: Adapting the Pentax FA*85/1.4 onto the Cantax K-5D Mark III