Pixel samples are really better if you use an "odd" number rather than an "even" one. The reason is because the engine takes a sample on either side of the centre one, so obviously an "odd" number is more efficent.

Also your minimum shading is way too high, should always be below 1, usually anywhre between 0.5 to 0.1

Injustice will be avenged.
Cofiwch Dryweryn.