So I just finished playing some on my Sony XBR55X850D, HDR both on and off. Whatever input lag was introduced with HDR on might have been barely noticeable, but it was negligible at best, and certainly didn't hamper my enjoyment.
The HDR itself however didn't impress me. It just made the colors more muted and dimmer, it was like the exact opposite effect that Sony showed off in that Horizon demo, where the colors POPPED. This, the sky kind of looked perma-cloudy. I turned HDR off, and suddenly the sky and ocean were vibrant blue again, and I like it much more.