AMD's Tonga GPUs have been on the market for over a year but it appears there's still no full-power version of this chip. Everyone assumed early rumors about a 384-bit memory bus were incorrect as cards like the R9 285 and R9 380X feature a 256-bit memory bus, but now AMD confirms that Tonga really was 384-bit memory bus capable all along.
PC Perspective heard directly from AMD's graphics head Raja Koduri that Tonga has a 384-bit memory bus, but they couldn't find the right price/performance slot in the market and decided to lock the Tonga parts to a 256-bit memory bus. Launching Tonga with a 384-bit memory bus meant cards with more expensive 3GB or 6GB GDDR5 memory and it seems AMD didn't think it could position these competitively.
So does this mean an upcoming Tonga GPU could offer this wider memory bus? Tonga itself was a follow-up to Tahiti (R9 280/280X), which did have a 384-bit bus, but all along the choice had been made to keep the updated core at 256-bit.
Now more than a year after the launch of Tonga a new part featuring a fully enabled memory bus doesn't seem realistic, but it's still interesting to know that significantly more memory bandwidth is locked away from owners of these cards.