Thanks for the advice guys! Reading back on my earlier requirements, it feels a little like I'm wanting the moon on a stick! I should probably keep the old triple constant in mine - cheap, fast, good - and pick two.
Weight - I can probably just man-up and deal with a slightly heavier camera. At the moment, my main interest is shooting wide and using my polaroid back. The Wisner does have a bail-back but it's just a little tight to fit the Polaroid back comfortably. I have had it working and it does fit, just tightly. I guess I could always remove the back and slide the Polaroid back in to stop any issues of knocking the camera out of focus.
Anyway, there's definitely some ideas for me to think about, thanks very much!