Pedro, I have played around with that HyperSync crap and a leaf shutter for years and never got any usable results. Few people actually get it to work properly above 1/320 and if so, mind you, you don't gain any light because the leaf shutter will limit your flash just as much as it limits ambient. I'm so glad I switched to central shutter! Compared to my old 5D, my flashes now have 3 stops more light at the same ambient light (because I can sync at 1/1600 instead of 1/200 and open the aperture 3 stops wider).
About the focal length: you basically have two crop factors when coming from a DSLR: 1) that of medium format compared to 35mm and 2) that of the back compared to full 645 frame (most backs are smaller). Calculate the diagonal of the actual sensor area of the back you want to get and divide it by the diagonal of a 35mm frame and you get your crop factor.
About sync speeds: yes, the older backs have 1/800 native xsync with CS lenses while the newer ones have 1/1600. Ask your P1 dealer about the upgrade options... if you want to upgrade to a better back later, you can probably give back the old back for the full price when you get a better one.
You might like this article by David Hobby where he writes about the setup you're interested it (scroll down to "Ditching..."):
http://strobist.blogspot.de/2012/01/bai ... on-d4.html