Huh I forgot to hit Submit. sqiddster For such a large area, I don't think the performance would be any different doing it on a layer vs doing it on a big object. And the moment you have a second object on-screen with bumpmapping, you're now having multiple shader calls instead of one for the layer. It's almost always on a layer in my game since I have multiple objects with it applied.
And also, I agree the shader could use an overhaul. Firstly, the Y axis is still inverted so light is shown from the wrong direction vertically. It works fine for scrolling and layout scaling. I haven't tried anything more advanced than that, so I haven't run into the other issues that shinkan mentioned.