Thought I'd give a little update to this thread 6 months on, still very happy with the scope
The two pictures are curvature maps with an FX sensor. The first one is two images in CCDInspector and the second contains a 10 (so theoretically more accurate).
As can be seen, very flat and well corrected over an FX sensor
The back focus distance is optimised for Canon DSLRS (44mm) over the Nikon (46.5mm) but the actual back focus distance is quite liberal but I have been using a MUCH longer back focus with my QHY22 (17mm camera+17mm CFW+24mm spacer) and not had any issues. This was of course pushed to the limit recently (~25-30mm further than optimum) which lead to a fair bit of field curvature but I have a new adapter on the way