It may have been built to NHK requested specifications, but everyone else benefits too. Compared to the F800, the 850 has improved CCDs, uses less power, weighs less, can take a Triax/Fiber adapter, has a faster Ethernet connection, can accept an optional Wi-Fi/cellular adapter for recording H.264 proxies and sending them wirelessly, has GPS built-in, and has a better LCD panel.Thats god to know, now is the 850 essentially an 800 with a few upgrades to accommodate what NHK wanted or is it a whole new camera? My limited understanding is that it had been pretty much made for NHK.
So the 850 is really an upgraded F800, not a revolutionary new model. Nobody who owns an F800 is going to trade it in for an 850. But for people buying a new camera, the 850 has some improvements over the F800.