Some quick thoughts: You've got Defer Atmo enabled (as it is by default in TG4), so you should not need such high atmo samples (32). You can turn that down to 16. You don't need Soft Shadows unless you've got shadows falling from objects onto objects, clouds are basically soft shadowed by default.
You can also use more highly adaptive AA with a low noise threshold, go to Edit Sampling, enable Customize Sampling, and you can set First Sampling Level to something more like 1/16 or 1/64, then the noise threshold down to 0.025 or even 0.015 (the lower the number, the less noise, but the longer the render). This lets the renderer use fewer samples where they're less necessary (mostly darker/less contrasty areas), and save time, while still using higher samples where needed. If you're going to animate this you may need to tweak the adaptivity differently though. Also if you're animating, you'll need to use the animation options for Cloud GI in the GI Settings window.
Anyway those things should help you cut down the render time a bit while maintaining quality.
- Oshyan
