Easy, open them in separate tabs, switch between the two and the much higher quality AO should be obvious.
Reasonable performance hit from experience.
If you have to compare 2 stills by flipping between one another to notice a difference then what's the point, as you'll never notice any difference when the game is in motion and your actually playing the game.
I really don't see the point in enabling such an unnoticeable effect that impacts performance, I would always take frame rate over graphical effects...especially ones that can't be seen easily