Production companies often adopt similar methods to reduce the rendering times and retain the overall quality.
It is worth pointing out that Ambient Occlusion (i.e. AO) as a separate pass is utterly imperative for the final shot.
The following exercise will take you through another unique methodology of achieving similar results with reduced rendering times:
...
jamiecardoso-mentalray.blogspot.com/2009/12/mental-rayadding-glow-to-window-glass.html