k-5IIs is pretty impressive. I think comparing it to D800 might be a stretch but I don't think there is anything in APS-C world that beats it. Handling, focusing, and general fit and finish are just excellent. Plus weather resistant and IBIS and small size. Almost too small, I wish it was a quarter inch taller so my pinky would fit.
I don't know about 6400, my tolerance for noise is low and I rarely shoot over 800 ISO, but certainly it is competitive with far more expensive cameras in this area.