Just to add to the mix, I currently use an IQ260 and find the image quality to be superb. I do shoot on a tripod always for my professional work and photograph objects that don't move (buildings), so my experience with the back on professional shoots is limited. Also, my main camera is an Arca Swiss RM3Di. Pretty much I am almost always at 50 ISO when working with clients (no reason not to be), and the files are great. Occasionally I am at 100 & 200 ISO when working with people and the files are pretty good as well.
400 ISO is really where you start to see a drop in IQ. I would say still usable for some applications, depending on the project and job. 800 ISO really drops, but once again you can get an usable file depending on what you are doing. You also have sensor + mode, which can help too. With that said, sensor +, although great when it came out, has pretty much been out dated by CMOS sensors.
I got some great street shots with the IQ260 on the XF in Cuba when I went a few years ago. I was typically working at 200 ISO to 3200+ ISO when shooting handheld. I found all of the images to be usable for this project. If I had a gripe here, it would not be the IQ, but working with such a large heavy camera for 12 hours at a time. If P1 came out with a mirrorless camera similar to the Hassy, I would probably buy it and stop using the XF for handheld projects.
The long exposure mode on the Dalsa chips (those that have it) is at 140 ISO. Although this will mitigate noise due to heat on the sensor, you still get noise from working at an ISO above base. To compensate for this, I typically over expose my images by a stop to a stop & half when working in the long exposure mode and then pull them down in post. This brings the quality of these files back down to that of base ISO but with a loss in dynamic range and, occasionally, the highlights can start to look funky.
Last, I did test out the IQ4 150 last week and was very impressed with the back. Not so much for the resolution (to be honest I could care less), but with how the back handles lens cast on technical cameras. I would say that the Schneider Digitars will see a rebirth in usage when this back becomes more widespread in use. Also, with my wide angle lenses, using them without a center filter with a CCD back would add a lot of noise to the corners, but with this back, the noise was controlled a lot better but still there. Additional features make this back more attractive as well.
However, if you are not in need of great lens cast control, very high resolution, or reduced noise at high ISOs from a reduction in files size allowing for faster shutter speeds (like with landscape shooters), the 150 may be over kill for what you are doing.