2.2x is perf/mm.
2.5x is perf/watt.
Here is the most recent TPU review. The 270X is about 5% faster than the 270 so based on the 1440p results the 270 would be about 106% of the tested 950.
Multiply by 2.5 for perf/watt and you get 265% which is a few % behind Fury X. Add 10% and multiply by 2.2 and the performance is between Fury and Fury X.
At 1080p the 270 would be at 100% so 2.5x is 250% which is ahead of Fury X. Do the perf/mm calc and you get 242% which is also ahead of Fury X.
So ballpark figure of around Fury X seems right based on scaling from a similar sized, similar TDP, same market segment card.
It might be a bit less but that is still > 390X perf. The idea that the top version will only be 390 perf is pretty daft IMO.