This was posted by doorqyu over at CDFreaks.com forum. On BenQ Korea's website, you can d/l the latest offical firmware from BenQ (B7S9). Like the B7P9, it will support 4X DL & will improve the writing strategy for [AN31] DVD-R media. Included in the zip file is an up-to-date media list for both DVD-R/DVD+R/DVD+R DL. So far, the new firmware seems to be doing better than B7S9.
Here is a direct link to the firmware which zevia posted: http://service.benq.com.hk/HKUploadFile/Drivers/ZIP/DW1620_Firmware_B7S9.zip
Edit: For those that want it in the .cvt extension, Brother Vlad has posted it up this version as well http://tdb.rpc1.org/original/1620B7S9.ZIP