I don't think you can find a BenQ unless you find it on a site like eBay. You'll have to search quite a bit.
I also do not think it is possible to write faster on DVD-R media if it is not enabled in the last DW1625 firmware. You can try to crossflash to DW1620 (16x DVD-R support was apparently enabled for it), but you will lose LightScribe.
Drives as old as the DW1620 and DW1655 do not handle many DVD+R DL well. It was almost always required to use Verbatim's MKM 001 and MKM 003 media. It is only with newer drives (maybe from 2007 or 2008) that other DL media could really be used without major failure.