I've got a TEAC in an USB 2.0. external unit, that is basically an OEM liteon. I've upgraded it's firmware to 1693S. I've been burning DVDs for a year under XP without any problems.
After installing Vista I noticed that I got really crappy transfer rates from the unit. I installed Nero and ran DriveSpeed. I noticed that my unit was limited to 6X. I manually increased this to 8X, which is now the maximum for some reason. Under XP I could select up to 16X.
To sum up: My external 16X USB 2.0. DVD burner is limited to 8x for some reason. Device manager shows no errors, everything else is working perfectly.