No one knows since no one has seen a UHD projection system in a theater. Remember, UHD is still just a spec, and a few prototypes living in some labs around the world.
UHD is 7680 pixels wide, versus the 2K pixels (width) for the current digital IMAX projection system (which is only deployed in multiplex cinemas with smaller screens).
Note that traditional film-based IMAX systems effectively render about 6K pixels. The original shooting stock is 65mm film passing through the camera horizontally for a bigger film gate, 69.6 mm wide at the normal 24 frames per second. Comparing digital projection to film stock is not an exact conversion, but basically, good commercial film stocks should provide about 80-90 picture elements per millimeter which is where that estimated 6K pixels number emerges.
What UHD will probably initially do is provide high-quality high-definition digital projection on larger screens where right now only film-based projection systems can provide the highest resolution. From a content standpoint, getting UHD source material digitally will probably increase the type of content available. IMAX cameras (the ones used to film) are noisier than conventional cameras and recording dialogue is difficult. Hence a lot of IMAX films are of the nature/landscape style that don't require audio. Also, it is possible that UHD cameras will have better sensitivity to light and perform better in darker environments.
There is a organic quality about film that adds a pleasing texture. While that can be duplicated in part using digital methods (digital film grain options have been in 3D animation packages for 15+ years), it is hard to say when the digital film grain will catch up to analog film's.
Again, it may be difficult to see the difference in UHD and IMAX unless you understand all the myriad factors in creating these films. You could be watching a UHD film and thinking "this doesn't look any better than IMAX". Of course, you might be watching some somber dialogue scene in a candle-lit room, something that couldn't have been recorded on traditional film-based IMAX cameras. There's a lot more than projection display resolution when comparing imaging systems.
In addition to your good post: The new movie, The Hobbit, is being shot at 48 frames per second. According to Peter Jackson the sharpness of each frame makes the overall viewing experience much better as each frame has less blur due to capturing less motion. The viewer is getting much more of the available sharpness of the lens and film with only a change in the frames per second.