This is the last time I trust GAF "insiders" or "experts" who proclaimed this to be false or impossible respectively.
No one even developers thought they will deliver 8GB of GDDR5. Frankly it is still unknown how they did it. (Stacking ? And if yes whats with heat?)
I believe also that 8GB of GDDR5 ram was super late to include (probably this or last moth) so most insiders wouldn't even know any of it.