The bottom line is that Scorpio's six teraflops will almost certainly go a lot further than an equivalent PC part. I asked Microsoft about this specifically, and they raise a number of good arguments that make the case strongly. Firstly, that their shader compiler is far more efficient than PC equivalents (think of shaders as native GPU code). Secondly, addressing the hardware directly via their API and with access to console-specific GPU extensions again adds to the advantage of a fixed platform box. And finally, they point to their optimisation software - PIX (Performance Investigator for Xbox) - as a tool that provides the path to console-specific optimisations that PC simply cannot get.
From what I've seen so far, there is some evidence that Scorpio's true 4K performance could pose a challenge to the likes of Nvidia's GTX 1070 and AMD's Fury X-class hardware. I've seen Microsoft's new console running a Forza Motorsport 6-level experience locked to 4K60 on the equivalent to PC's ultra settings - cranking up the quality presets to obscene levels was one of the first things developer Turn 10 did when confronted with the sheer amount of headroom it had left after a straight Xbox One port. Out of interest, we tested Forza 6 Apex with similar settings at 4K on GTX 1060, 1070 and 1080. Frames were dropped on GTX 1060 (and a lot of them when wet weather conditions kicked in), while GTX 1070 held firm with only the most intense wet weather conditions causing performance dips. Only GTX 1080 held completely solid in all test cases. It's only one data point, and the extent to which the code is comparable at all is debatable, but it certainly doesn't harm Scorpio's credentials: Forza 6 Apex received plenty of praise for the quality of its PC port.