Thanks pradipta and neha
I did discover something though. Carrara is a bit different compared to other renderers when it comes to GI settings. You have photon counts and interpolation, photon map accuracy, caching, etc. But for lighting quality which amounts to the final shading and how the other settings are taken into account, there are just a few settings. Preview which is quick to let you see what the scene will look like, just with noise.
After that you have Fast, Good, Excellent, and Best. There is also a setting for pixel accuracy for the lighting. Going from 16 down to 1. Unless you have many tiny details though, 8 to 16 works just fine for most GI scenes.
The good part is that I just discovered that Best was not really needed, at least on the AO render. I dropped it down to Excellent and it rendered in 11 minutes and 16 seconds. Over twice as fast and there is no discernible difference between the original AO image I posted. Not to my eyes anyway. BTW both images are straight from Carrara.
I am trying the Full GI version with it set to Excellent to see if it does ok. I have the feeling that because GI is more demanding, there might be a noticable difference in quality compared to the Best setting.