Tonga isn't Tahiti based, you can't tack on new gen features onto an old gen core. You're talking about a ridiculous amount of work and cost in taping out when after tahiti everything moving forward will be work on newer version of the architecture.
Only a couple of websites more recently are saying rebrand and tahiti based, those articles also literally got every single piece of information wrong. Somehow when I pointed it out the person posted on here, saying they'd change it, said it was based on a bad translation(because GCN1.0 is often mistranslated by google into GCN2.0... and just made up the story again but at least getting some of the basic information right).
It's twaddle, at worst Tonga will be GCN 1.1, it will be similar architecture to Hawaii and people forget that Hawaii has significantly improved performance/watt to Tahiti. It's possible and maybe more likely that Tonga will be GCN2.0, with further performance improvements.
From the relative leaked info(aggregating it all together) you generally hear new features and better performance efficiency, neither of which come from rebrands... ever. You can't take the same core and just make it lower power. The things that make newer cores more efficient is redesigning it for newer/different processes(which happens at the same node, there is more than one 28nm process at TSMC, and Global).
If you're also getting 280 performance into a smaller card, it's an almost certain indicator that it's a smaller die that requires less power and less circuitry on the PCB, another sign it's a new more efficient architecture.
One article in which literally every single piece of information stated in it was completely inaccurate(calling 7700 2 generations old when it's GCN 1.1(the current generation), calling Tahiti GCN 1.1, saying two features were exclusive to R3xx series cards but that Tonga was a rebranded Tahiti... yet had these two new features. It was all complete nonsense.
As said, bet on at worst GCN 1.1(but still tweaked for better efficiency compared to a 7700), but more likely GCN 2.0.