The extra $100 off was enough to convince me to pick up the 55KS8000 from Best Buy yesterday, and after a quick calibration (modified version of Rting's settings) I'm extremely happy with everything so far. While the actual dimensions are almost the same size as my Panny 46PZ800U, the extra screen real estate is fantastic (not to mention that it weighs probably less than half as much).
I haven't explored any 4k content yet, but I watched a lot of basketball and streamed content from my PS4, and I am incredibly happy with the upscaling. Great black levels on Game of Thrones. I would occasionally notice a little light bleed along the top edge, but only when I was really looking for it; I didn't find it to be distracting at all. The Smart LED adjustments were slightly bothersome though, not sure if that's something I'll keep set to "High."
As far as gaming, I just tried it out for about 15 minutes to see the difference between Game mode and normal, and I'm very happy with the performance of Game mode thus far. Everything feels appropriately responsive, though I didn't do any extended playthroughs. I'll try some twitchier games this week and see how it responds. The input lag on Normal mode is brutal, so I'm very glad for Game mode (though I wish there was an quicker way to turn it on/off; maybe a Harmony macro?).
I'm very happy so far, though it's only been about 6-7 hours. I'll put it through its paces this week and see how it goes.