Thank you to everyone for your input. I used the GXR again over the last few days, and although it is slow and I miss the occasional consecutive shot, the image quality is lovely, and I'm reading about corner smearing with the X-Pro 1 and M lenses.
My only real gripe with my M9 is just a function of the camera itself - no liveview. The ability to use liveview to compose and expose is something I find really useful, so I hoped that a X-Pro1 with its faster shot to shot time, rangefinder handling and liveview might be a good solution. Seems not, if GXR images are qualitatively as good or better. Thanks again to all of you.