Hey there,
I've been working on a desktop - web player project so I haven't tested this on mobile. I think the shader should be pretty fast though.
Actually, this method uses alpha tests which has a deep impact on PowerVR's GPU's, especially SGX one (used in
a lot of mobile devices) as it discards the texture cache of the cell being rendered. This should be really slow for example on iPhone 3GS / iPhone 4 / iPad 1.
For other mobile GPU's I don't know.
