Hi, I've some things to add for you:
First, can you mention that as a plus, BenQ 1620 based drives are exceptionally good readers? I've been able to read discs that none of my other drives could deal with. A downside of this though is that they're not ideal for disc quality scanning, as they give much better results than other drives will.
I recently got hold of a Philips DVDR1640p drive, so I can add this info:
The dual-layer 16x DVDR1640p uses the same hardware as the BenQ 1620, and thus can also use BenQ 1620 firmware. The BenQ utilities also work with this drive, even with Philips firmware installed.
Note that only the retail versions of this drive have the nice large LEDs on the front - the OEM (bulk) versions only have a plain fascia!
The current offically released firmware for it is P2.4 (although P2.5 is also available, I've uploaded it here). The imminent P3.0 firmware will add 4x dual-layer support, and 16x DVD-R writing (ie. it will catch up to the recent BenQ firmwares). There is no RPC1 patched Philips firmware that I'm aware of, but RCP1 BenQ firmwares can be used (see below).
This drive can be cross-flashed to BenQ 1620 firmwares, with the appropriate .CVT file (using official BenQ firmwares will not work, as they will detect that this is not officially a BenQ drive - .CVT files don't perform this check).
However, the BenQ firmwares disable parts of the Philips retail LEDs: 1) the blue DVD media LED no longer lights - instead, the CD LED is used for both media. 2) the large blue 'read' LED no longer lights (the red 'write' LED still works though). For this reason you may want to stick with Philips firmware (I do : ).
If you want to go back to Philips firmware after having flashed BenQ's, you need to find a Philips .CVT file. These are hard to come by at the time of writing. I have uploaded P2.2 CVT here (thanks Chesko). Once this is flashed, any official Philips firmware can be flashed again.
It is a good idea to flash back to Philips firmware if you need to send the drive back for replacement/repair. Flashing unsupported firmware voids the drive's warranty.
That's all for now.