I have an Aptus 75 and Canon 5Dmk2 myself, and worked with a Hasselblad 22 megapixel back, studied lots of raw files of various cameras, including the Aptus-II 5 and its older incarnation Aptus 22. The image quality of Aptus 22 and Aptus-II 5 is virtually the same, and its not strange as the same Dalsa sensor is being used, so yes its "old" technology. However, CCDs became good already back in 2004 so the improvements up till now if we only look at low ISO performance is mostly in resolution. Yes dynamic range and color rendition is a little bit better in modern sensors, but the difference is so small I consider it irrelevant for real picture making.
At base ISO and in scientific measurable terms the D800 sensor is a little bit better per pixel than the Aptus-II 5. Canon cameras is still behind. However, the look the Aptus-II 5 will produce is different from the D800 and many prefer the Aptus-II 5 look. I am myself the type of photographer that's quite agnostic: as long as a camera records reasonably accurate colors with reasonable dynamic range the rest is post-processing. I use MF for the cameras I get to play with, not for the sensor in the backs.
The greatest weakness of the Aptus-II 5 I'd say that it's 22 megapixels, which makes it quite prone to moiré and color aliasing. I can live with that, but some find it really really annoying and want higher resolution to reduce the problems, just going up to 33 makes a difference (although you still have some of those problems). In technical photography shooting at f/16 is a good working aperture, and with ISO25 you'll notice that you will have quite long exposure times. Usually not a problem, but you it will be considerably longer than for a D800, especially if you use wide angles with center filters.
Unless you're very into the MF software workflow and look the real gain in getting the Aptus back is the cameras you can use them with. The tilt-shift lenses for D800 is very limited compared to the range of lenses and flexible movements that exists for a technical camera. I use a view camera myself and I think you should consider that for your studio work. A second hand 4x5" geared Sinar camera could be a solution for studio work and a "pancake" camera for your landscape work, or look into getting a Linhof Techno which I use myself and think is a really good field camera for the ones not scared of ground glass.
Note that with 22 megapixels and f/16 as working aperture you can't fail even with poor ground glass
. Pancake cameras tend to get rather expensive if you want tilt and many lenses (high lens mount cost).
Weather sealing... hmm... I currently have a support issue with my Aptus, it's in for repair a second time. I can just say that I personally don't trust the product fully until they've fixed mine. The problem I have is that it fails now and then in cool temperatures (+4C and lower). Many have success with them though and Leaf claim that they should work, but all I can say I'm having a bad experience and I've seen a few others have reliability issues when asking around here and other forums. If it's 1 out of 50 or 1 out of 5 that has problems I cannot know. I think its safe to assume that these are not as good as a pro-level DSLR, but if you get "a good copy" it should work in well in most conditions including very cold temperatures, and it does have some minimal sealing despite the vents - it's not as bad as it looks. Just talk clearly to your dealer about which conditions you are going to use your back and say in advance that you will return it if it does not work as promised. And when speaking about dealer, pick a large dealer with good reputation concerning support. Here in Europe many dealers are small, only 1-3 people and have trouble keeping up support. If you want the best reliability from MF in tough conditions the P+ series have the best reputation.
If you're going to buy new as it seems you are, then make the dealer work, really show you the product and the workflow. Then you will also get a feeling for if it's a good dealer to have if you run into problems later on.