As I'm getting quoted as an "experienced photographer" I might as well update my take on this. My first P45+ was indeed broken. The new one can take images *by streetlight* which would be suitable for full-page magazine use.
I would say that ISO 1600-2500 is now quite decent, although my first impression is that 400 on the old broken back was in some ways smoother. The bad press that MF backs have had for low-ISO seems less due to noise, than to various other issues like banding which can vary a lot from sample to sample; all backs seem ok at ISO 100.
I can handhold the Mamiya decently to about 1/20, with an 80mm, and the huge sensor does make up for a lot for the remaining noise, compared to something like an M8. Also lights in the frame retain coloration, which is a big plus in night and available light images. Where my Canon would really beat the Mamiya is with the fast and sharp F1.2 lens - it's more the choice of lenses that creates the camera speed than the sensor. Reading the specs shows the P45+ sensor performs much like the one in the Canon 20D, which was quite good for its time, and I's day compared to APS you can tack on at least another stop for the fact that you have so many pixels to average over.
Maybe my complaints seemed overloud at the time, but I do find it interesting that after all those lectures about underexposure, swapping the back got me one that actually performs pretty much as I expected
PS. As this is an Oly thread, let me say that what the Oly really lacks are a couple of fast tiny lenses, eg a 35 equivalent a 50 and an 80. Drop those in your pocket and you have a nice backpack camera, which a lot of the original Olympus SLRs were renowned for.
This impression is reinforced when experienced photographers like Edmund present seriously underexposed P45+ shots at ISO 400 and 800 and complain about noise in the shadows.
Here's one such ISO 400 shot from Edmund which looks to me as though it's as much as 3 stops underexposed. Although the amount of EC correction shown in ACR is pushing only +2 stops, in relation to ETTR one should be counting from minus 1EC and possibly less. An ISO 400 shot underexposed by 3 stops is equivalent to using the camera at ISO 3200.
So what we're really seeing here is a P45's attempt at ISO 3200. Not bad really .