I don't have experience with the Pentax M* 67 800mm 6.7; but I do have experience with the Pentax M* 67 400mm 4.0 and Pentax A* 645 600mm 5.6 on a Pentax 645D. So some of this may be good information for you.
Not sure how much you know about Pentax lenses but you don't want the old 6x7 lenses that do not have a star designation. This primarily only leaves the Pentax 67 M* 400mm (which I have) and the Pentax 67 M* 800mm. Stay away from the 6x7 400mm 4.0, 6x7 600mm 4.0, and 6x7 800mm 4.0. Aberrations and sharpness are not up to digital... The 645 A* 600mm is an excellent performer but is not up to a 1.4x converter. Ergonomics are fantastic on this lens. The 67 M* 400mm 4.0 is a fantastic performer and is up to a 1.4x converter; however, the ergonomics are not as good. This lens does not have the top handle and requires an adapter. Still up in the air which is my favorite lens since the 400mm is the optically best performer but the 600m is the usability performer...
Now for the medium format vs dslr discussion. Well before the d800 a dslr did not even cross my mind. It came down to in most circumstances I got the same quality image as was coming out of 16-20mp dslr with my 40mp medium format; sharpness, focus, higher iso, motion, etc. resulted in an image that could only be printed to that size and not show image imperfections. However, on that rare 20% occasion you yield an excellent image. And thats what it was all about using the medium format. However, now with the advent of the d800 the game has changed. Sure, it is still a battle to pull the full resolution but there is more in your favor with the dslr and image stabilized auto focus lenses. I would have a d800 now to make a decision if I wasn't living in Namibia... But I am. Now on the other hand I like the 6x7 and square format so a digital medium format is a good ratio to crop to that...
Anyways, on to the use of long lenses on medium format. First I find I have to have a focus magnifier to obtain crisp focus. Second a solid wooden tripod is best, just seems to work better than carbon or aluminum. I also use the really right stuff parts for the long lens support with an additional clamp going to the mount on the camera. Then a 4th leg like the manfroto lens support which goes from the camera down to the tripod to further stabilize. Weight the whole thing down with bean bags... Setup takes a lot of practice to get everything setup outside, nail focus and get everything locked down followed by mirror up... Third you have to have decent light or bump the iso higher than you want as with that amount of resolution you need to be using fast shutter speeds. I found a drop in polarizer was a life saver here because you could shoot a little later in the morning and a little early in the evening when the light was bright enough. It helps keep you from loosing contrast... But again takes light away... Shooting from a vehicle was difficult, possible but difficult. Shutter speed and movement inside the vehicle must be watched very closely. Vehicle must be turned off as is common sense...
Now on a side topic less than 5% of my wildlife images are shot with the long lenses. It just has to have a lot working in your favor to pull it off. I rather have better luck with a 300mm and incorporate much more of the landscape. It works well here as the landscape is amazing that we have animals in. The long lenses are used for about 10% of landscapes (primarily the 400mm) as it is a unique perspective creating some nice abstracts... However, that leaves to massive and expensive lenses used for less than 6% of my photography. I can't help but think I would use the longer focal length perspective more with a d800 as more things are in your favor.
Anyways, I think that is some good information to help you. Ask any questions you want, I have been using the system for two years now on everything from lions, elephants, antelopes, landscapes, etc.