I wonder about the same thing!
QScan tells me that i cannot safely burn verbatim 16x DVD-R (mcc 03rg20) at any speed (from 2.4 to 16) on my BenQ 1640. i used to different sets that i obtained from 2 totally different sources, so i don't think it's a bad batch (they are both made in taiwan though).
the results are alright though, but they seem not to be as good as those as other people get with the same burner/media combination.
i have not tried other media yet, i start to wonder whether the burner may not be alright...
so what are your experiences with qscan?