Thanks guys, some useful comments above.
I'm waiting on another GI pass to kick off a render over the weekend. I've Adjusted some cloud settings to make the cumulous brighter as well as adding another cloud layer to bleach out the horizon. I'm going to see what more I can do to the surface textures today and tomorrow, but as this skybox is intended for camera orbits of an aircraft a lot of detail will be lost to motion blur.
@Oshyan, the only post work on the image above is from the default settings of the HDR Toning in photoshop from converting the exr to 8bit. I've set up a skybox in AfterEffects which rotates in accordance to a camera from Max. The sides have been set up to work in Linear colour space, with layers for Surface RGB, AtmosRGB and AtmoAlpha, which I'll use to grade the final composite.
I'm very much liking the GI cache feature. In this instance it allows us to split each side into smaller crops to render but maintain consistant GI when stitching back together.