given that the earlier scattering issue (scatter errors due to colour) and the displacement/scattering issue have been recognised as bugs, I can understand why users wouldn't want to try increasing IDL samples, as it increases render time by approximately the square of the sample size.  in the past, users were opposed to setting IDL IC = 100 for a similar reason, but the latter variable may only shrink blotches and not eliminate them. 

however, increasing IDL samples for a small area render in a spot known for displacement blotches might not be prohibitive, assuming the displacement blotches always occur in the same spots in duplicate renders that use IDL IC < 100.