Top Left is the reference/start image
HDR moon-bluegrey.hdr (Horo)
Render time 28:34
Top Right I've dropped the Quaility to 0 this has introduced a lot of shadow banding but look at the render time (also note how the scene has got slightly lighter - this is as a result of the sampling of the hdr to determine the brightness of the simulated light sources - higher quality will even out the effect of dark and light areas in the hdr whereas lower levels of sampling are a bit hit and miss - so be prepaired to make adjustments)
Render time 1:11
Bottom Right keeping the Quality at 0 I've added a bumpy terrain and added a bumpy texture to it to try and disguise the shadow banding
Render time 3:56
Bottom Left again with the Quality at 0 I've employed a chequered material on an infinite plain to break up the percieved banding and added 25 reflection to further confuse the eye
Render time 2:19
Conclusions... well to be fair this is highly subjective but I'd say that using reflection and pattern was the most effective way of "getting away with" lower quality in IBL.
Oh perhaps I should have said before... the render times are minutes:seconds.