The closest equivalent to PlayStation 4 Pro's GPU in the PC space, in pure performance terms, is the Radeon RX 470 or
an underclocked RX 480 - both mainstream graphics cards. The latter product typically overclocks to hit the same six-teraflop compute output as Project Scorpio. However, teraflop comparisons here simply do not reflect real-world results. With one or two exceptions, the 'next-gen' upscaling techniques used on PS4 Pro don't tend to exist in the PC space - and software-level PC upscalers that
can produce great results still only manifest in a minority of games. On both counts, we expect things to change rapidly, but the bottom line is that right now, consoles are capable of producing some great 4K results even if the technical sleight of hand required to get there means that it's not strictly speaking a 'proper' native ultra HD resolution. This means more bang for your buck from your console purchase.
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 Inspector 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.
The bottom line: PC games need to evolve to more efficiently address 4K, in order to bring down the cost of GPU hardware to effectively and consistently power an ultra HD screen with the latest games. The best console titles have led the way here, and it's a tradition we expect to see continue on Project Scorpio. If third-party games live up to the first-party results we have seen, where 900p and 1080p games scale up to native 4K, the comparisons with PC hardware will prove absolutely fascinating. If Scorpio's GPU can hold native 4K and hand in results on par or better than GTX 1070, this is a seriously good result for a console.