Maybe, but that's gotta be awfully difficult to test on XB1. Just seems odd MS would update to code that neither of their consoles currently support when trying to bridge PC and XB.
It should be backward compatible but to what degree is up in the air right now. The premise of what MS is doing now is different than in the past, they are trying to make their new API's and shader model extensible just like their OS, no more get the full package type deal, so things are changing to a more like Vulkan type additive features. The outcome can be anyone's guess right now.
The fact that future HMDs are looking at 120Hz would seem telling and that's down around 8ms frame times. So without some interesting adaptations we won't see any progress on VR hardware as they simply spend all their time running the same content faster.
Well I'm not going to presume things on VR since I haven't looked into it much but from a human body perspective yeah, there are many other things VR can possible do with higher frame rates and lower latency which haven't been discovered yet. Unlike traditional monitors there are many variables that are just unknown at this point.