To my experience, BenQ does fairly good burns at 8 to 12x burnspeeds, especially on DVD+R media. Looks like BenQ engineers been concentrating on quality in this burnspeed (media) segment.
2.4x burnspeed, seems to be a forgotten brother in BenQ 1620p firmware...
But my NEC 3500 beats BenQ quality at 2.4, 4 and 16x burnspeeds, on most of the media I'm using.
Not only this, NEC burns them faster at same choosen burnspeed.
Finally, not all old media is bad media. I have some 1y old 4x speed SonyÂ´s (RICOHJPNR01) that burn better at 8x on all my burners then any nowdays 4x media.