Just thought I would post a couple of photos at iso 6400 with K5 and K3. They aren't perfect, probably, but I think they give decent comparison of the two with noise reduction turned off as much as you can in Lightroom.
K5:
K3:
I apologize ahead of time for any differences in focus, framing, or other things that may make it difficult to compare these photos. The K3 crop is a little tighter than the K5 crop, as well, for what it is worth. Hopefully it gives an idea of the two cameras performance comparatively at iso 6400...