I guess it’s for CD-Audio that you are interested in lower jitter?
While I can understand your concern, I have to say that despite being among the best DVDR burners around, the Benq 1640/1650/1655 are not top-notch CDR burners, and you’ll never get beautiful 7% ‘flatliners’ with you jitter curves whatever the speed or the media used. These burners have obviously been optimised for DVDR (and bloody well at that ).
If you feel like you need low and constant CDR jitter, look somewhere else. You can get a dedicated CDR burner from Plextor or LiteOn, or a NEC DVD burner which are great CDR writers (but there is much controversy about recent NEC DVDR burning qualities, to say the least…)
AFAIK “best of both worlds” (DVDR + CDR) sadly don’t exist in a single drive, as far as jitter is concerned. But if it’s for data (hence low jitter is not important), as others have mentioned above, any recent burner will do fine at all speeds.