According to PCPER Tonga (not just Tonga XT) has been 384-bit capable all along and never enabled by AMD which prefered to release the thinner version because it never found the right price/performance combination.
AMD's Raja Koduri confirmed Tonga's 384-bit bus existance and it might mean that the next Tonga GPU could offer the wider memory bus.
PCPER does not feel that a year after the launch of Tonga a new part featuring a fully enabled memory bus is not realistic, but it's weird that AMD shot itself in the foot by making and then locking away significantly more memory bandwidth from card owners.
AMD's next-gen GPU will launch in mid-2016 with HBM2 and on the smaller 14/16nm process.
According to Tweaktown the new cards will be powered by the faster HBM2 technology and go over a 1TB/sec memory bandwidth. This is nearly double the 512GB/sec found on the Fury cards, powered by HBM1.
Tweaktown said that AMD's new cards will launch on the 14nm/16nm FinFET process, with the GPUs including a huge 2x performance per watt increase over the previous generation cards.
The move appears to be part of AMD’s spinning off its GPU business into Radeon Technologies Group and the changes to Radeon Software.
The Greenland GPU will be the range’s flagship with will feature up to 18 billion transistors, and up to 32GB of HBM2.