just to say, i really don't think it's the GC i'm running into.  not that i've tried turning off GC yet, but i'm seeing much more predictable results with just IBL.  i just don't see why a 2% infinite should make formerly dead black shadows about 7.5% grey (different place than previous numbers, in an area less lit by the infinite), but i'm going to try a few things to see if i can find out more.