Second to this: as a guesstimate, could the PS4 push Windwalker/Journey level graphics at 1440/90?
Re. some of the GDC Morpheus demos 'running at > 1080p in some cases', that might be a reference to MSAA (higher res depth buffers etc).
I think Sony will target 1080p @ 60/70/75 ... I don't think 90fps. I think they may trade latency for higher refresh via interpolation if it's not good enough for flicker free low persistence.
I think Ground Zeros is a nice timely example of 1080p/60 on PS4, but I think it's also really only the beginning of what devs who take care will be able to do.