Well, it is possible.
The 6800 has the 4 pipes disabled, are they disabled because they are defected of just because they wanted to make the 6800 card is currently unknown.
There was a review where they flashed the 6800 BIOS with the 6800Ultra BIOS and the rest of the pipes ware enabled but they showed some glitches in certin games altough Far Cry ran just fine!
The problem was that after the "backflash" the pipes remained 16!
Why are the pipes remaining at 16 and not going back to 12 is unknown and so is the reason why some games show problems while others run flawlesly, we need to wait and let people play with it some more.