BTW, I rendered it @ detail 0.7 and AA4 with raytraced atmosphere (RTA) enabled.
I suppose you're using the free version of TG2?
If so, then I'd recommend to render with detail 0.8, RTA enabled, AA3 and GI 1/2/8 as I don't think this scene needs high GI settings.
With detail @ 0.5 the GI will be 0.5 x 2/2 = GI 1/1.
With detail @ 0.8 + GI 1/2 the GI will be 0.8 x 1/2 = GI 0.8/1.6 which is similar as above, but faster (likely).
As for RTA and AA3: set atmosphere samples not higher than 24, usually, and keep your cloud quality setting around 0.7 max.
If that is not sufficient you can either choose to adjust AA3 to full sampling instead of the default first 1/4th, or increase samples slightly.
Cheers,
Martin