There's been a few posts on this, so I thought I would share my experiences after buying a consignment of 90 minute CDs.
Firstly update to the latest firmware, for this model it is OS0K. Using the latest version of Nero, set the overburn time to 98:58 mins (don't know why?!). And make sure that Write Method is set as Disc-At-Once. When prompted, say you will Overburn the disc.
I found that the fastest speed I was able to write at was 8x. According to this website http://www.medeainternational.co.uk/info/90min.htm the most you can put on these CDs is 90:05, but I haven't tried that much yet.
Well, that's about it. Laters.