Corona is an unbiased renderer. Realism through many bounces, reflective caustics active by default is the norm with such engines.
So, engines like these are mostly used where one needs an HQ photoreal look without much adjustment. Corona, for example, is by intention very easy to use.
You don't need to change any settings, the defaults are working well in pretty much any situation. Also, it won't become much faster by reducing the number of bounces.
You can, however, set the GI mode in the generals tab to only use direct light or a single bounce. But those settings are usually for special cases or for debugging.
About AO: Afaik, there's always a little bit added to the scene, but AO is (roughly speaking) just an emulation of GI and not needed when the GI solver has done its job well.
You can assign a Corona AO shader to your materials or add it to just one and use that as an override material in the generals tab. (Just make sure that in the current version, every object in the scene needs to have a texture tag) In that case, you can change the GI settings, though it should render quite fast without maps and use that output in PS to enhance the shadows or mask out certain things where you used the AO shader. Though, I'd use the "render only masks" option or a selection etc. pp. for such a thing.
->Interactive Documentation<-
