Yes, for the BenQ it could be a SolidBurn thing, this was mentioned in the thread also. We'd need to see a burn each with SB on and off then. Can you do this, Arachne? Maybe this can be achieved with the "Simulate" option in Create Disc (all we need to see is the (virtual) burn graph, not the quality scan)?
Now, if that should solve the question for Verbatim MCC004 on a BenQ 1650, it still leaves the issue about the LiteOn 1635S with HP CMC.MAG.M01 not burning at 12x or 16x either. Still curious whether it's strictly a burner/media issue or something in the system.