interesting to know there are a number of people here with the full Sinar M system. The problem is it's a system waiting for the right back to exploit its amazing AF and lens quality.
The backs exist, (P65+) the complete Sinar M system exists, but the two aren't talking to each other, which is just ridiculous.
Sinar - if you want to make money, and sell cameras, then open your platform up. simple!
The right back to really
exploit the Sinar m is still the Sinar 54H, at least in the studio. In 16-shot mode, the P65+ cannot hold a candle to it. I have mentioned this fact several times on this forum. It is because the 54H can physically capture 88MP each of Red, Blue and Green. The P65+ is a single shot back that captures 30MP of Green, 15MP of Red and 15MP of Blue, and then uses software to estimate the remaining 120MP of needed pixels. There is simply no comparison when this much information is estimated.
Even for non-tethered work, I do not agree that a P65+ is needed, unless you depend on heavy cropping (ie throwing away a lot of captured pixels) or print really large. The Sinar eMotion 75LV is more than capable. If you compare actual printed images on a 11500 or 9900 printer to see where the advantage of the P65+ kicks in, you will not see any difference in the prints until you start going significantly beyond 24x30. This is using Zeiss FE glass (e.g., 110FE, 300FE) with the P65+ versus the Zeiss Sinaron digital glass with the e75LV, to be as fair as possible. And, I am being incredibly generous here in the print size, since even at 30x40 you may still need a magnifying glass to try to argue over any final print details between the two. With this much money involved, I strongly recommend renting these systems and doing the test yourself.
If you are shooting hand-held, then you will really see nothing gained by the P65+. I don't know anyone who can shoot hand-held and be able capture 50 lp/mm. The mirror slap on any SLR is enough to blur such fine details (with any MFDB). As a result, you get more pixels but not with more resolution. So, if hand-held AF is your primary method of shooting, then the P65+ is not delivering any extra image information compared to a back with lesser megapixels.
To be fair to the P65+, I think its strength is mainly when you are in situations where you cannot use a multishot back, and where you can shoot slowly and with careful technique to extract any resolution gains from it, AND need to print very large (or crop very heavily). Otherwise, I find the Sinar m system is the perfect balance for my needs, whether it be for fast AF with Digital Zeiss lenses, or as a shutter on a p2/p3 system with Digital Schneider/Rodenstock lenses. So far, I have not found any other camera/back that can produce a better print (up to 30x40). Again, there is no substitute for your own tests. Give them both a try and you may be surprised.
Lastly, I totally agree with your plea to open up the Sinar m interface!. This will give people opportunity to compare the backs easier and see for themselves how many megapixels is enough for their prints.