If it just happens that you have the FA 35mm f2, I'd love to hear you comment on the difference in IQ. As I understand it, the 35 mm is a very good lens too, so I wonder if the IQ difference can defend the price difference for an amateur like me. I do need a fast lens shorter than the 50 mm for indoors.
Photozone might be able to answer this for you soon: http://www.photozone.de/8Reviews/index.html
I noticed that they reviewed some Pentax lenses recently already.
From my experience, it is close to FA 50mm F1.4 in character and performance. Which is not bad at all yet it might not be top notch. I noticed some chromatic aberation (lateral?) of a green variety in some shots I took this weekend. Not a big deal for my needs.