I think it could be made to look better if it took into account the surface normal of the fragment (something like less distortion in x/y when the surface is pointing at the view point in x/y, so in other words use the x/y components of the surface normal in camera space as a scalier for the distortion in those directions) that way as that giant metal chunk swings around it'll have all sorts of interesting movement of the background image when the cloaking effect is at it's height. also camera space depth could be a useful scalar so things that are closer, maybe would have the background enlarged or something.