I shoot a Sony FE 35mm 1.4 Distagon on Sony system and that is what I consider bitingly sharp wide open.
I've found that the FA31mm limited has a tendency to front focus a little on the K1 - I've had to dial in +3 on my K1 with focus tune and lens align. When I dialled this in, I found a huge difference in wide open performance and wide open is very sharp.
However in context, the FA limited is not nearly as sharp as the Zeiss distagon wide open and I shoot both for very different purposes. Where I want clinical wide open precision I use the Distagon. If I want artistic 3d pop rendering and creamy bokeh I shoot the FA limited. The two lenses have very different design goals from different eras.
However by F2.8 and up the FA31mm sharpens up considerably to be practically as biting. That being said the Zeiss Distagon to my eye does not render as nice the out of focus areas. Quite literally the FA limiteds make subjects jump off the background!
This is not to say that the FA 31mm F1.8 is soft on a digital Pentax - it's not. Results will appear soft in bright high contrast out door situations wide open. It depends heavily on the lighting that you are shooting in (it is susceptible to coma wide open and fringing).
The 31mm has been optimized for a film plane rather than a digital sensor. On film it is probably one of the sharpest lenses out there.
Where the FA Limited comes into it's own in my opinion is how all the little deliberately designed 'imperfections' - coma/field curvature/vignetting - come together to create a very artistic organic looking image that pops far more than the 31mm focal length would suggest.
The Bokeh on the FA 31mm is creamy smooth - more so than the Art or Distagon 35. It has plenty of sharpness wide open but lends itself towards a 3d pop.
This is pretty close to out of camera with a 20 second black and white conversion in LR, no sharpening. Open full screen to see sharpness.
Evan by
tom.ohle, on Flickr