I have a NEC 2510A and it burns those Ritech 8x ink-jet-printable disks (Ritech R03) at 8X no problem (using Nero 6.3.1.20). It also burns Ridata 4x disks (Ricoh R01) at 6x...
I've never had a problem playing these disks on the computer or my two DVD players (a VERY cheap Apex player and an expensive Samsung DVI player).
IMHO, you are just going to have to wait for a firmware update (and NEC doesn't seem to supply firmware updates on their website)...