According to our sources, AMD is still working on the R9 280X based on an updated Tahiti GPU, the Tahiti XTL, which should be ready in November and most likely replace the Tahiti XT2, currently used on all Radeon R9 280X graphics cards.
According to our sources, the Tahiti XTL GPU was simply not ready for the launch and all current Radeon R9 280X graphics cards ended up with the Tahiti XT2 GPU instead. Various sources confirm that AMD will do a, so called, "running change", where there will be no announcements and no name change for the new GPU and the only way you would be able to tell which GPU is behind the R9 280X is by its BIOS version.
Of course, nothing is carved in stone as AMD can simply decide to launch a different named graphics cards with Tahiti XTL chip but currently the plan is to do a running change.
Apparently, Tahiti XTL should have a lower power consumption and thus should be easier to cool. We will surely keep an eye out and our ear close to the ground in order to spot the new GPU in November.
Source: ocaholic forum