Okay, here are the final results of my external enclosure odyssey:
Although I ordered an Oxford911 enclosure they first sent me one with a GeneSys711 combi chipset (the shop did swap it very fast and at no costs for me - two thumbs up!). Put the 822A+ with B3HC in it and it did work out of the box via USB2 and Firewire.
Yesterday the Oxford911 arrived - USB2 still worked fine (GeneSys or Cypress, dunno) but Firewire didn’t (drive not recognized by WinXP) - arrrrgh
So I thought, wtf and started flashing the Oxford firmware (I know it is dangerous) to version 3.8 - still no go and then played around with the Oxford settings (6s delay, recognize non-standard devices, etc.). Good news: Didn’t screw up the enclosure but still no drive recognized via Firewire (I did recognize an old HDD just fine btw).
Next thing on my list was the BenQ firmware. I flashed B3EC (I think) which I got from a webpage posted here somewhere. I did warn me about the downgrade but flashed just fine. I did flash via USB2 because I was somewhat pissed at BenQ anyway
After a reboot the drive was recognized via USB2 and Firewire.
I was still in experiment mode so I flashed the original B3IC which I downloaded directly from BenQ and to make sure I end up with a ruined drive I flashed it via Firewire g. Well, to my big surprise after the reboot the drive was recognized via USB2 and Firewire. Amazing!
Summary: GeneSys711 worked out of the box for me, Oxford911 worked after some weired flashing and I am happy - excellent burner, excellent enclosure, long story though