Has there actually been any proof of new dies other than Fiji existing? Plenty of proof about Fiji, starting from that 550mm^2 leak last year but I've yet to see anything else that would imply a new die. Anything remotely reliable that is.
Also the stuff about Fiji being a lower end GPU with HBM (4GB) and bermuda being a higher end HBM one with (8GB), the theory that raghu at least bases his speculations on, seems to be most likely false.
http://cdn.videocardz.com/1/2015/04/Hot-Chips-Symposium-Fiji-2.5-HBM.png
^ If AMD had a higher end GPU than fiji and fiji was going to be used for the 380X instead of the 390X they wouldn't be holding a presentation about fiji in august. They'd be doing a bermuda presentation.
And the rest of the lower end leaks for mobile and for desktop have showed reasonable proof for rebrands.
Even the leaked XFX double dissipation card had a pcb that was identical to hawaii PCBs from everything we could actually see on the card.
Personally I think that AMD's architecture improvements over tonga are limited to nonexistent and as such it didn't make any sense to design any more GPUs which would be only slight improvements over the old stuff, but expensive to design.
We'll likely see Fiji with almost identical GCN to tonga that gets its extra performance from the power savings from HBM, added die size and added space on the die caused by the change in memory controllers.
This is a stopgap generation anyway. The real deal will be when 16nm rolls out, both manufacturers are on HBM and we'll see which architecture actually is the superior one.