Pretty sure the max resolution is 4096x4096. Thats kinda unnecssary though, as you could just break up the elements into smaller textures while still retaining 1080p if your going to use geometry.

There dosen't seem anything wrong with the theory but I have no idea how you will animate the textures when they are clicked on/hovered etc

p.s i think you might be wrong about the ui elements, ive seen some UI's that definitely look nice at 1080p??