Let me share two comments:
*I do not own a K-5, but I have a K-7, K-01 and K-3. I worked this week in a basement with my K-3 + FA31 mm and I was impressed by the high ISO and AF performances of the setup. Definitely much better than with the K-7 and K-01.
*In very low light, I believe that we are best using MF lenses. Having work a lot with the K-7 (known for poor high-ISO performances), the K-01 and K-3, I strongly think that fast MF lenses are the best way in very low light, although I fully appreciate that there are situations when AF is preferable.
In low light conditions, I shot many indoor and outdoor situations with my K-7 + MF lenses, when others (with Canikon + AF lenses) would stop. In one case, a guy with a Canon FF and AF lens could not autofocus and gave up at sunset, when I was still shooting with my K-7 and VL58mm f1.4 (MF).
Hope that the comment may help you and others...
Last edited by hcc; 06-20-2014 at 08:08 PM.