a little bit of both i think. it must be taken into account that the media is only 8X certified so burning at 12/16X is out of spec and it's hard to complain when poor burns result from overspeeding the media by twofold.
that being said, my 12X burns are fine and others have perfectly fine results at 16X (in both the PX716 and other drives i.e. BenQ, NEC, etc.).