"Complete specs are still unknown, but rumors say, that the GDDR6 RAM could be faster than HBM2."
Provide evidence.
By what measure?
Are you comparing super high-end overclocked GDDR6 in labs, to the HBM that's in professional/consumer products today.
Current HBM 2 chips can run at higher clocks than to ones AMD put in the RX Vega cards.
RX Vega only has two stacks of HBM 2 too, double that and you massively increase the card's bandwidth (whilst also increasing the price).