Based on the footage I've seen, it should be running at 1080p :/ The X1 is perfectly capable of maintaining a stable 1080p/30fps in a game with SO's graphical fidelity and action.
That said, Insomniac's North Carolina studio don't have a great track record when it comes to performance optimization (Nexus ¬_¬), so it's probably for the best. I'd rather a stable 30fps at 900p than 1080p with the frame rate doing a little dance.
Another possibility is they might have optimized the whole game around the performance requirements of the co-op mode (which looks more active than what I've seen from the SP). That's a perfectly acceptable excuse, but if that's the case then it would be nice if they added a variable resolution. No need to hold back the SP for the MP.
