The drive's firmware examines information on the blank disc to determine what speeds are claimed to be supported for burning. These speeds appear as bold face in the speed drop down menu in the Recorder Settings menu (only, of course, after the disc is inserted and recognized).
Hmm, nevermind. I did an actual burn of about 720MB to a DVD+R on both my PC and my Mac.. both came out at 2.5 mins. It's just that Nero on the PC reports 16x speed which probably isn't accurate.. or Toast is wrong and it's getting a faster burn speed. Either way, it looks like the drive is consistent with my 16x NEC burner on the PC.