I don't know if this is part of the problem or not and how much ?
Anti-Aliasing and Lens Distortion
If the anti-aliased method used is FXAA, it is now performed to smooth-out the jagged edges of the meshes.
Then, to simulate a real-world camera, a lens-distortion is performed on the image by using a small pixel shader. It does not only distorts the image, it also introduces small chromatic aberrations on the edges of the frame, by slightly distorting more the red channel compared to the green and blue ones.
It might very well be that this as no negative impact on the visual if done right, only a direct comparison between 4 and 5 lens distortion would reveal this.