Actually, the GSA-4120B and 4160B writers did support DVD+R DL writing, both at 2.4x. The GSA-4082B did not, unfortunately.
firework : TheDangerousBrothers are the people who actually create these modified firmware releases you keep hearing about. If they are telling you that firmware cannot do the job to give that writer DVD DL writing capability, they know what they are talking about.
From what I know, there's modified firmware which does various things with writers, but I have never seen modified firmware to add DL capability for the GSA-4082B. There is a limit for what you can do with firmware. If the hardware is incapable of doing it, there isn't much you can do.