I decided to pitch the new XF14 (on the X-Pro) against my Zeiss 21 (on the 5D2) in order to evaluate if the Zeiss was worth retaining.
Please note this is not scientific just inquisitiveness on my part!
Using a Sekonic 758 lighmeter as reference, I set both cameras up on a tripod and shot from f/2.8 through f/11.
I noticed that the X-Pro at ISO 200 was ~1/3stop underexposed compared to the 5D2. I also noticed that the XF14 was not quite as wide as the Zeiss 21mm - maybe closer to 22mm (35mm equivalent).
Wide open, the Zeiss showed significantly more vignetting and distortion (moustache like) than the XF14.
Running the raw files through Capture One and adjusting for WB, exposure and vignetting differences, but not correcting any distortion or sharpening, I would say that from a pure detail point of view (microcontrast and resolution) the Zeiss 21mm on the 5D2 still has the edge - although only by a small margin.
However, the X-Pro and XF14mm performed much better than I expected, given its compact size, sensor format and price difference. Colourwise, I actually preferred the slighly warmer tones of the XF14, although this could be adjusted on either in post.
I have since decided to keep the Zeiss 21mm as one never knows what FF Canon may be around the corner! I also have a feeling the the Zeiss is not fully challenged by the 5D2 sensor and may offer more. Whereas the XF14 is probably close to its limit.
For reference, hi-res and size matched CS6 converted jpegs (zipped) shot at f/2.8 can be found.