This TV upscales really well. I swear my OTA TV antenna feed (1080i) looks nearly as good as native 4K content with a clean program on (I've never seen Steve Paikin look so REAL until I got the X900E, hehe).
I'm really happy with getting this over the P series Vizio (plus having to add some crappy tuner box as it has no tuner) due to it's epic processing of lower resolution sources. It also really does OTA justice in other ways, the guide button is 100% functional using OTA and gives me a complete and accurate schedule of all the channels it picked up in the scan, and even tunes into channels that my previous Vizio could not which seems to show that Sony didn't completely cheap out on putting the cheapest tuner they could find into it either.
I can't really say how games translate at lower resolution, the only game I've been playing on it since I got it has been Destiny 2 and I've been playing it at 4K with HDR enabled so everything has been native there. I just wish now that I had ponied up the extra cash for a GTX 1080 Ti over the GTX 1080 now that I've had a taste of true 4K/60hz gaming on the big screen.