The enclosure is a couple of years old, but in the last week I have updated the 911 chipset to the latest firmware. And the BenQ is set as master too. Plus I'm using an 80 pin cable.
I was wondering if it might be something to do with having poor quality Verbatim media, but I've also burned a few cheapo discs that offered up to 8x, but still took the same 20-odd minutes.
Speed is not that important to me, so if I could get to around 8-12x with the current setup I'd be more than happy. Rather than buy a new Prolific enclosure I think I'd probably just go for a second burner, perhaps a Pioneer (since they are a bit more Mac-friendly), but ideally I'd prefer not to have to buy any new hardware.