I bought 8 CDs BENQ certified 48x… when I wrote them I have a big surprize: the first sectors and the last ones were perfect written @ 48x but IN THE MIDDLE (min 30-40) they were plenty of bad sectors…C2 errors uncorrectable. My writer usually performs fine onTDK, Sony, Ritek etc media but once I encountered such a problem I was stunned. I hope this was an isolated case but I thought it would be wise to warn others.
The curve of errors you mention is consistant with over-speeding the media. Regardlesss of the speed rating, you can usually comtrol this by slowing the burn speed. Not the first time that media has been re-labeled with higher speed ratings.