If you would ask Plextor Support why it would not really support overburning, then this will be the response that you will get:
90 and 99 minute CD’s and overburning are outside the specification, and may not work correctly on all CD-ROM drives and CD-R recorders. 90 and 99 Minute media use a tighter spiral track on the disc to get the additional recording time, but many drives don’t read this properly. Overburning uses space normally taken up with the leadout area of the disc, and not all drives will read the last track on the disc correctly if this leadout area is not used.
For that reason, Plextor does not officially support overburning or 99 minute media, although we do nothing in our hardware to prevent it. If software is capable of overburning and the media supports it, our drives will write it. However, some CD-ROM drives may not be able to read the media because of additional errors from trying to read beyond the limits of the CD specification. Plextor Manager 2000 1.04 and later does support 80 minute media.
For help with 80 minute discs and Adaptec Easy CD Creator, see: http://www.roxio.com/en/support/discs/disccompat.html
ECDC and Plextor Manager don’t support 99 minute discs at this time.
Plextor Support Team - Paul