We have sold a number of P3 systems with non-Sinar digital backs and they work fine. Sinar does have a very good sliding back adapter that accepts V-Series Hasselblad mounts, although there is a new adapter insert that will change that to H-Series Hasselblad mount. It's solid, well constructed, and features a lock as well as a toggle wheel to advance (gasp!) a Hasselblad film back.
KaptureGroup also makes a fine sliding back adapter for P3 that accepts V-Series, H-Series, Contax 645 and Mamiya 645. Not quite as nice as the Sinar, but half the price.
I believe when I've put P3 standards next to P2 standards, they are slightly shorter and finer tuned than the P2 standards. I can verify this Tuesday. However, we have many customers who saved $4,600 by converting their P2 to P3 instead of going for the whole system. The conversion kit costs $3,650 and replaces the P2 frames that attach to the standards, as well as adding the smaller bellows. Otherwise, it's $8,211 for a new P3. For medium to long lenses, this is a reasonable path to pursue. For wider lenses, the precision and smaller size of the P3 will be an advantage.
Using non-Sinar lenses that are wider than 60mm will require a recess board, which Sinar does not manufacture for the P3. SK Grimes does have such a board - in fact we approached them with the initial project - but these boards work either with a Sinar sliding back, or a KaptureGroup Video Mount, because the Sinar Sliding back replaces the rear frame, and the KG Video Mount closely matches that focus plane. We found that you cannot use the KG Sliding Back with the deepest SK Grimes P3 recess board because it seats the digital back too far past the minimum focusing plane. This is a up to 35mmXL Digitar. We have not tested the SK Grimes recess board with the 24mmXL Digitar yet.
Sinar just did come out with copal shutter mounted lenses for P3 which will eliminate the need for an SK Grimes recess board, if you buy their lenses. The Copal shutter mounted lenses are a bit less expensive than their electronic CMV and CAB lenses.
The biggest advantage of going P3 and all Sinar (back, lenses, everything) is that you have complete integration, one supplier, computer-based operation, etc.
But that doesn't mean other digital backs don't work just fine. For someone who values what the P3 offers, rock-solid stability, great system accessories, all the features one would want from a view camera - yaw-free, self-locking, zero detents, etc, it makes a fine platform for digital capture.
Dealer for Leaf, Hasselblad, and Sinar