Yes BenQ released a firmware update that allows burning at DVD+R DL discs at 4x but only for selected media and I think the only one supported at 4x is Verbatim [MKM] discs (not sure if this has changed with the new B7T9 firmware).
Yes the Philips 1640P and the 1620A are the same drive chip set wise just different externally.
I can only speak in reference to NEC drives as I have been a NEC user since the start so what BenQ offers that NEC doesn't is;
Good consumer support always got a reply to any question I sent to BenQ.
Out of the box support for Bittsetting on all DVD+ media types
Official support for over speeding media something that NEC owners must rely on custom firmwares for.
Disc quality checking tools for before and after burning.
Good firmware support with regular updates to support new media and firmware upgrades for older drives so that they become the new models.
e.g 1600->1620; 1620->1620 Pro
Nice face plate for the drive with multiple colours well I think its better than NEC's at least though getting the non-beige one ani't always easy.