According to the latest we are hearing now, Ryse: Son of Rome actually is native at 900p, but it is upscaled to 1080p using the power of Xbox One. Actually, all titles that don’t run in native 1080p are upscaled to 1080p by default.
If you actually would take the time to count the pixels, the difference in quality from 900p to 720p is a big difference. However the difference between 1080p and 900p actually isn’t that much. You are not going to have a lot of quality loss between the two.
No change on Forza 5 as it is native 1080p at 60fps.