I have found that my best performing lens on my XP-1 is the 55mm micro Nikkor. Up to 5.6 it outperforms my 50mm Summicron, which is the other lens I use in this range. It may be bettered by the the Zeiss Planar 50 for Leica M, which I tested very briefly and not in a controlled environment. I understand that it does not respond to your needs, but I do not see much point in using a very fast lens on the X.trans sensor, being high ISO so good. If you want blurred backgrounds, I suggest that using a longer lens may be a more practical solution. It brings an advantage: 85-105mm lenses have alwas been some of the best products offered by the industry. On the contrary, 50mm very fast lenses were not supposed to be used at full aperture (apart from specialized f/1.2 lenses, like the Noct-Nikkor, sold today at fantasy prices). The f/1.4 was supposed to help people to focus decently in pre-autofocus times, and have some headroom at better apertures, which used to go from 2.2 down.