Its very unlikely now that the BenQ1640 has been released with -R DL support. Of course there is no real need as the -R DL format is hardly compatible with any devices at the moment as it cant have its booktype changed. I think the +R DL format is the one people will want to use. Of course you never know BenQ support their drives extremly well and they could release a firmware update to support -R DL in the future but you can never tell.
P.S = Has anyone tested to see if the BenQ 1620 will read back -R DL discs properly?