See Performance tips
[quote]Size, angle or opacity of sprites/tiled backgrounds has no effect when hardware acceleration is in use.[/quote]
Generally textures 1024x1024 or smaller should work on all devices.
Forget the power-of-two thing.
Performance depends on the system, you need to test on a range of devices.
Only things which are visible on the screen affect the framerate, unless you have really intense events or complicated behaviors like lots of pathfinding going on.