I like to use a GI light for surfaces and, if it's not needed I just drop its Strength in Atmosphere down to '0'. That speeds up renders by a huge amount, it's good for bright daylight scenes where atmosphere shadows aren't so important.
Sometimes, though, you won't want to do this. Disabling GI in atmo is really, really fast but maybe you need some atmosphere Envirolighting, you could try with any fill-lighting to brighten the atmo, another tip that some people liked when I posted it before was: Use two Envirolights. Disable Atmo Effect on one(GI), disable Surface Effect on the other(AO). That will give you envirolight for both areas, at a much cheaper cost. AO comes out very strong in atmo and surfaces, try setting the respective Envirolight(AO) strength to about '0.1 to 0.25.'
While I'm posting lighting tips, not strictly GI related but. Soft shadows can take a long time to render, especially in heavily atmospheric renders. Chances are, you won't see the benefits of high SS samples on your surfaces because you'll hit 'Stop' on the renderer because it's taking too long, the atmosphere is likely making use of your Soft Shadow samples, that you're unlikely to ever see shadows from.
Simply duplicate(ctrl/cmd+D) your sunlight. On the original sun, uncheck Lights Atmo.
On the new one uncheck Lights Surfaces and Soft Shadows.
That should speed up some rendering, a lot!