I'm not that surprised that the DF is that much better than the K-3 at that high of ISO levels. The DF (and D4) is better than a lot of full frame cameras when it gets that high, whereas K-3 seems to be about average for current APS-C crop at ISO 3200+. If there's a reason to buy the camera, besides the retro looks and compatibility with old Nikon lenses, it's that you can get that class leading high ISO performance for less than $3,000.
Still, for me, the DF in this incarnation is a miss, even though a lot of things about it appeal to me more than the other Nikon full frame cameras out now (which is why it's selling well, I imagine, despite being polarizing). If it had a great viewfinder they'd probably have got me with it, despite some other quibbles I have with it, such as it having a 1/4000 shutter.
Thanks for sharing your thoughts on it, though! A part of me certainly wishes I had the best of both worlds like you do