I doubt ATO is going to have a significant impact on headways. There might be some minor impact if they include automatic door operation. The looming budget crisis due to low ridership is going to dictate headways/levels of service more than ATO.
I do look for forward to smoother train rides, especially not having operators lurch the train to a stop to avoid platform overruns.
As for Rosslyn I don't remember WMATA ever going below 2 minutes with ATO. I believe the switch is around ~130 seconds to set.
It's probably the case that this was good intent given the lack of desktop ARM computing hardware, but they really should let the client decide the video quality.