Well I really like mine, dont know how it compares to K20D as I upgraded from GX10.
AF speed is fine, depends on lens, on my Sigma 50mm 1.4, faultless,............. Sigma 70-300,.............not as nippy, but no complaints.
Low light, have only played around so far with it and its miles better than the GX10, shoot RAW, slightly to the right. have all noise reduction in camera turned off and tweak in Noiseware, gentle on the sliders and everythings good.
I have found that (well for me anyway) that having everything pretty much turned off, or set to zero, enables me to adjust jpegs easier, with RAW files available for a more detailed approach.
Loads of inbuilt features, some really useful, some not so.
Thought the d300 was ok in the low light stakes ............
Dave ....