I got some very cheap DVD +R DL disks from Microcenter on monday - $10 for a pack of 10. I got 20. They were sold as 8x DVD +R DL.
In my Plextor PX-750A with the latest 1.03 firmware, Imgburn said the disk only supported 2.4x speed

I downloaded MediaCodeSpeedEdit and DVDIdentifier to patch the Plextor firmware. After I pasted the Media Code Block from DVDIdentifier into MediaCodeSpeedEdit, I was able to produce a firmware that gets these DVD +R DL disks to burn at 6x . This is not bad, but I would like to get the full 8x.
Does anyone know how to do that ?

Recommend you check the quality of the burn using CDSpeed or similar program. If the 8x vendor rated speed was limited to 2.4x in the drive, it could be because 1) there is no write strategy for that MID code, so it defaults to 2.4x, or 2) the media writes crappy in the drive so the firmware limits it to 2.4x. Getting higher speeds in exchange for unusable discs is not such a good tradeoff.

Bob,

I believe it is because there was no write strategy for that media code that the drive only burns at 2.4x.
I will check the quality of my 6x burn with CDspeed.

Assuming if it is good, what can I do to make the drive burn at 8x on this media ?

Well, the media doesn't seem particularly good. I had 2 discs crap out from 8 that I burned. Not sure which drive they were burnt in and at what speed as I burned some in my Sony DRU-700A at 2.4x and some in the Plextor at 6x, simultaneously and from two different hard drives. I should have run the verification as I normally do on single layer but it takes a long time on double layer. I'm going to do some more burns and track it more carefully this time, but probably I won't be buying this media again.

Isnt the Ricoh dual layer media manufactured by Ritek and therefore similar to the 2,4x Ritek dual layer disc?

I have no idea, but what I know is that the media isn't worth the cost. I had bought two 10-disc spindles. I returned the unopened one. Of the other, a good half of the discs went to the trash, when all verification was done. Some wouldn't even complete burning.
Better to buy Verbatim media that works and save time ...