Why do you expect the RX Vega drivers to be significiantly better. They have 1 month to optimize, when they have been developing the drivers for 9-12 months minimum, likely more.
Well we don't know if the architecture is a complete overhaul because the dudes at AMD won't say anything about the GPU... So if NCU is to GCN what Zen is to bulldozer, it could make a difference. It is not logic that this gpu underperforms compared to Fiji AND Polaris, at this stage even Polaris has a better or on par IPC.