So in a studio, is there a big difference between a D800/5DMkIII and a 645ADF/II and the Aptus 22mp back? I am looking for images out of the camera with minor editing.. I would like skin tones, sharpness, etc to be as close to natural as possible.
I rarely work in a studio, but I use a Sony A900, Nikon D800 and Hasselblad H3D, so my experience may be of some relevance.
A word of caution first. Basically, if all one does is photograph test charts or measure sharpness, dynamic range and color accuracy, one will find out that 24x36 cameras work very well indeed. But this is also a consequence of how the test is done: by reducing all variables to the common denominator, one will necessary find commonalities.
If you are taking pictures of real subjects and are "looking for images out of the camera with minor editing", you may find out that MF images are generally more pleasing. Most noticeably: caucasian skin tones are more pleasing out of the camera and bokeh is generally smoother. The difference is not like day and night, but it is noticeable to the eye.
You should also not forget that there are differences beyond sensor size:
-MF has a much better viewfinder
-MF has a much poorer AF: accurate but slow
-aliasing is a problem on MF backs
-MF top iso range is quite low
-size and weight (obviously)
-lack of zoom lenses (or it is even more size and weight)
-MF lenses are generally very sharp but relatively slow. The availability of very fast lenses in 24x36 allows it to produce thinner depth of field than MF, BTW (contrary to the popular belief).
(the following two do not concern the 645AF, which has a focal plane shutter:
-MF has a higher flash sync speed, which is useful with outdoor flash
-MF has a much lower top shutter speed)
Also: before you buy a 22mp back, I strongly suggest to check the used prices of more recent backs or the price of a new Pentax MF camera. Prices have dropped in the past months.