I believe the image processor (signal amplifiers, noise reduction circuits, etc) has more to do with the IQ when it comes to ISO performance. I don't think Sony would have actually designed different 6MP sensors. Rather, the sensors may very well be the same (as in, bought from Sony in a huge batch), but they may be tweaked differently. There are other brands using the same sensors, but even the RAW data will be different because they tweak them differently, put slightly different UV/IR/antialias-filters on top, etc.
Still, I don't have any evidence they're the same, but that's what I got from reading around
