I don't know how people around the web can be so sure Vega will not be competing with the 1080ti / Titan Xp.
In my mind there are two ways to look at Vega (with the information from the latest leaks e.g a 64CU / 4096 Core card clocking in at ~1500mhz - 1600mhz) without taking into account any gains that have been made on how efficient the architecture is.
1) Basing on a Fury X at 1500mhz
2) Basing on a 480 with 4096 cores
I understand both of the above are not really scientific as the Fury X had issues with allocating tasks to the cores it did have and the 480's Polaris arch will not scale 100% with more cores. But then again we know that AMD has shown of some upgrades to Vega on paper.
By using the results from a 1080ti results here
http://www.guru3d.com/articles_pages/msi_gtx_1080_ti_gaming_x_review,29.html we see that the MSI Gaming X 1080ti gets around 23,000 in firestrike on defaults.
Using the Fury X results (14374) and its default clock of 1050mhz we can guesstimate that a 1500mhz Fury X would hit around 20500 (right between the 1080 and the 1080ti) (calculated on scaling the freq (result ÷ base_freq) * target_freq -- (14374 ÷ 1050)×1500)
Now using the RX 480 (2304 core)@ 1266mhz boost results of (11298) and guesstimate a 4096 core 480 @ 1266mhz would hit around 20100 again between the 1080 and 1080ti (calculated on scaling the cores (result ÷ base_cores) * target_cores -- (11298 ÷ 2304)×4096)
Now this is very non scientific but its about as good as what anyone else has argued against the Vega performing near the 1080ti or even the 1080. But from simple scaling we can guess that even if all of AMD's efforts to upgrade efficiency of the new cards fail (tiled rendering, better task allocation ect,) and we only get the performance per clock or core as the Fury X or RX 480 it should still bet somewhere between the 1080 and the 1080ti..... Or at least at the movement there is as much of an argument for that than Vega being slower than the 1080.