So after playing PAYDAY as potato for a day, I realized something. Even after following hagen's optimization guide, the game is dark.
So I modified the "Max Saturation" Color Grading that can be found in Options > Video Settings > Advanced

That way, it is makes it possible to have a much brighter PAYDAY experience even without shadows and in potato mode.

Side by side comparison can be seen in images.

1.0
Release