From my perspective, the P45+ and a tech camera has positive and negative issues.
Postive, the back will shift much better with lenses like the SK35, or SK43 or SK28, more shift with much less color cast issues. The chip is also void of readout lines, so you shouldn't have issues with tiling. The reason I mention the Schneider lenses, is that they have a lower cost of entry than the Rodenstocks.
Concerns, the P45+ has a terrible screen, and will not tether in the field unless you have a full sized laptop with a firewire port. As you don't have a good LCD to view your shots on the P45+ using a tech camera could get frustrating. The ability to tether via USB3 and use a Surface 2 or 3 is a huge advantage especially to a tech user. You might want to consider the Credo 40 or IQ140 for this reason. Yes the P45+ will do up to 1 hour exposures, and odds are better than the IQ260, but you need to remember that outdoor temp's can have a huge factor. You can only get a 1 hour clean exposure with the P45+ in 69 F or lower temps. If you try in warmer more humid temps then noise will really become an issue.
I used a P45+ for many years, and found on my back the iso range was 50 to 200 max, maybe 400 in bright light if trying to stop motion. But I could not push iso if wanted to use it in low light and use a shutter speed much past 1/15th of a second, it's just too noisy. I have seen images from other P45+'s that were shipped later in the life span, that seem to have much better signal noise image quality than mine did. I feel it's safe to say that during the manufacturing of the back, Phase continued to possibly tweak the back, or maybe some just come out better than others.
As pointed out by Dogs857 the shadow noise recovery is nothing like the D810, I would say, really nothing as I often push my D810 files 2.75 stops at base iso to iso 320 with no increase in noise. I never could do this with the P45+, even at iso50, instead bracketing was always needed. I also noticed a tendency to blow out highlights. Here again the more useable screen of the IQ backs will give you a much more useable display of information. Everyone will have their own view on this as no two sets of eyes are the same. It's safe to say, CCD loves light and performs best in good light.
You are working with a good dealer already so I would make sure to look at the Credo 40 or IQ140. These use the Dalsa sensor but have a 1:3 crop vs the 1:1 crop of the P45+. So 30% less than a full frame 54 x 40 CCD, and 10% less with the P45+.
Good luck with your decision.
Paul
. I've been talking with Dave in Atlanta about some used equipment. You are so right that I need to try before I buy. I understand I can process these files in LR? I never could make Capture 1 work and I gave it two tries. I'd really like to break out of the Canikon mold and yes, a tech camera would certainly be nice, especially since the t/s options for 35mm pretty much suck (and I tried them all). Perhaps I should be talking to the people in Atlanta about renting something short term and then I could do all my experiments at home. 