yeah, but the data transfer speed over that interface on the RXvega is 1.89Gbps whereas the 1080GTX is 10Gbps.

Memory bandwidth on the Vega does work out faster at 2048 * 1.89Gbps = ~ 483GB/s
and the 1080GTX is 256 * 10Gbps = ~320GB/s
(there is some overheads and other binary math factors).

That all said. Memory Bandwidth isn't everything. You also have deliver to the card, how fast the GPU can actually do anything with the data in memory, and a bunch of other factors.

In reality, the Vega 64 was disappointing for the money for gaming purposes (it is usually priced higher, but performance is a step behind a 1080 - a 1080Ti does even better). Apparently its quite decent for mining though.