You can also generate a sharp circular mask from the soft version, this is still less instructions than the material based method.
Personally I find it easier to create the soft mask in an image editor to exactly match the diffuse texture. A 512²px DXT1 map is only 170KB btw.
Each frame of your gif is showing heavy texture distortions, this is basically just stretched lines towards the center: http://abload.de/img/pupildilation_02rwu5e.jpg
