I did ask a question about what speeds were available to you for burning but never got the answer.
I understand that 2x is all you actually get but are you actually offered the normal speeds , like 4x/6x/8x/12x & 16x?
It does seem like you're only getting a PIO limited burn speed. This could be down to a faulty IDE cable. It's happened before to other members.
Try switching the Plextor to the primary channel & the HDDs to the secondary & see what happens then.
Also whilst DMA is enabled is the Current Transfer Mode actually UDMA 2 or 4?