The auto pre-scale of HLSL has a bug, that result in a much too small render target and you will see these blurry uneven scanlines; not to mention how a shadow mask looks. Therefore you should define a pre-scale which fits your vertical screen resolution and the vertical resolution of the game best.
e.g. Screen resolution: 1600x900 Game resolution: 320x224 Pre-scale: 900 / 224 ~ 4
You also can use higher pre-scales, but this will decrease your performance.
Anyway, I'm working to get rid of the manual pre-scale definition by using a correctly sized render target that suits the used resolution.