If you get the BenQ DW1640 you should burn DVD+R no faster than 12x because the burn quality on DVD+R at 16x with that drive is not so good regardless of media. Burns at 16x on DVD+R produce a "comb-like" effect in PIE/PIF scans.
I personally don't care much about high-speed burning anyway, since 8x is plenty fast for me and is the safest burning speed in most drives I have (6x is safer in some and 4x is safer in my NEC 4551). The time difference is not important to me, but the quality difference and the lower chance of bad burns are important to me.
So what I'm trying to say is that you shouldn't burn at 16x on DVD+R with the BenQ DW1640, but in my opinion that limitation is not a significant disadvantage.