I wasn't sure if B6 could use multilayer/exsposures HDRI, I guess it can. I was thinking of one of AgentSmiths posts about the size of the HDRI not making rendering times any different maybe. Oh yeah, I forgot about the quality slider making scenes light different, which you already found out.  Compared with other 3D app IBL features, Bryce does need to improve a bit, but DAZ has stated they are working on it now.

 Unless my eyes are going, I see the banding your talking about easy(poly smoothing in other app maybe?) plus artifacts and/or antialias compression(like .jpg) in your pic. I hope its not artifacts from IBL in Bryce. Im noticing this the more I use other apps and Bryce...