P2.4 is the latest firmware for this drive (supports mentioned media upto 16x), but as TimC said RITEKF1 is real crap on Philips/BenQ drives.
Bought one 10 pack once and all I managed with a DW1640 was 12x coaster burns.
BTW samplayford, your drive should support
SolidBurn and OverSpeed futures that can be enabled or disabled in
Nero CD DVD Speed (File-> Options-> Transfer rate).
Also, did you choose "DVD burn" in Nero as shown in attachment.