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.