Which distortion are you seeing? Is it the parallax? I have a lot of parallaxing going on, so there will always be some distortion with that. Also, If I do make a texture, I would not use the alpha channel. Alpha channels are uncompressed, and all you need is a soft gradient for the dilation and displacement map, and a sharp circle to define the SSS mask.
Also, if I do use a texture, I wouldn’t use the alpha channel: they’re never compressed, and cost just as much as a standard 3-channel texture of the same size. Since the parallax and dilation need a soft gradient and the SSS mask a sharp circle, I’d put both those masks in two channels of a brand new texture, and maybe put the iris texture in a third and just multiply it by a color. How’s that for optimization?
I’ll make a few versions of this shader and see which one is more optimized without ruining the quality.