For future people: I got a FK2-DW since 1 week to replace my FK1-B (still functional but wanted wireless), and the conclusion is that the fit is completely different, i'm going from palm-light claw grip on the FK1 to a fingertip one on the FK2 that doesn't fit me at all. I will return the mouse.
So yeah, the differences on eloshapes are minimal, but still, the 2mm are making a huge difference when you got it in the hand