I think that per-pixel lighting is unneeded. I think that it's a better choice to do lighting per several pixels and interpolate the result to per-pixel. In DX11 we can generate primitives by tessellation, and each primitive covers several pixels. So we can do lighting in domain shader for each generated vertex, and blend the interpolated lighting result with textures and other stuff in pixel shader.
I also have tried rendering to a smaller sized texture, 1/2 sized, and then enlarging it to full screen. The lighting was good but aliasing was too heavy.