Not that easy. The 600mm lenses for FF digital are autofocus and because of their high speed and the camera body's high speed allow for fast shutter speeds, which makes it possible to get reasonably sharp images under typical wildlife photography conditions. In addition, the mirror is significantly smaller than any MFD mirror.
The 800mm for the Pentax is "only" 6.3 and the back looses substanially at 1600ASA. This would all be o.k. for average lenses, but the 800mm/6.3 ED is quite a beast. Most medium format digital cameras add a lot of mirror and shutter vibration, and while these are acceptable for average focal lengths, at 800mm you will feel the effect, unless you do your best to minimize vibration (e.g. two tripods, bright sunlight, perhaps even other shutter dampening strategies).
Thus, those 600mm rigs for FF are likely the better equipment for wildlife photography, unless you are prepared to go to great lengths to reduce mirror and shutter vibration.
One lens that may be considered in this context is the 1000mm mirror lens from Carl Zeiss Jena. Obviously, the Mirotar will do as well. These mirror lenses are fairly compact and center well on a heavy tripod. They also have little chromatic abberations as they are mirror lenses. You'll have to put up with those donut shaped diffraction rings and that depends on your liking.
I have shot both the 600mm and the 800mm Pentax on MFD, and both are prone to vibration effects, even though both are wonderful lenses.
Just my 2c.