Well if you look at the Drives/Media section of this forum you can see the results yourselves, in my experience the faster you burn the lower the quality, look at Taiyo Yuden TYG03's burnt at 16x and then again at 8x, on all my drives (Pioneer, NEC & BenQ) they produce better results at 8x than 16 even on 16x TY's !
I have also found the same with the Verbatim DVD+R DL's, burnt at 2.4x you get good results across most burners, however we have seen people posting with problems when burning at 8x or even 6x ! For me, given the price of the discs I am happy to wait another 15-20 minutes for the burn.
That said your drives might be fine with the higher burn speeds on DVD+R DL's, thing is you will have to try it and see and then use DVDinfoPro to scan (BenQ and Liteon drives do this) the discs and obtain a quality score.