Shader optim ?

Hi, I have this shader that allow me to affect a sprite with some darkness effect as some kind of whirlpool and to display some light and dark area on a texture. But on iOS, this is a huge overhead for some device (iPhone4, iPad2) so I just want to know if there is a way to optimize this a little bit ? http://pastebin.com/KMYx9XWt Thanks in advance