I wouldn't qualify 2560 x 480 as "large", and multiple scenes would allow you to take advantage of atlases. I have projects with hundreds of scenes and have never had memory issues as a result. You can think of scenes as a container. Memory and CPU usage is a function of the scene's contents, not the size of the scene itself.
Granted, larger scenes do give you more space to put things. So you will want to be aware of what you're doing. Use good programming practices--don't abuse 'always' and 'drawing' events, use minimal physics actors when appropriate, etc.