The point of the second monitor is not for extra real estate, but to be able to use different monitors for different applications. You can only really concentrate on one monitor at a time anyway. And, when gaming or watching a movie, the last thing you want is a lit up second screen, next to your main display, distracting you - a bit like someone's lit-up phone in a cinema.
Doing some colour critical work? Use the colour-accurate monitor and turn off the gaming monitor. Watching a movie? Slide the monitors across so the gaming monitor's in front of you, then turn off the editing monitor. When mirrored, both monitors would display the same thing, so, no matter which monitor's in front of you, you can still alt-tab to access web browsers and other non-colour-critical apps.
Why does extending the display not affect performance, compared with mirroring? Two mirrored 4k displays renders 8MP and sends it to two different monitors. Extended, the card is displaying 16 million unique pixels. Not sure how that works. Either way, if you turned off one of the monitors, would performance return to normal? Or would the card still try to render and send data to the switched-off monitor?
Now, if only Eizo would bring out a 27" successor to the CG248-4k. The CG279X is only 1440p, while the CG319X doesn't mirror well, since it uses a different 4k from the standard (in addition to costing as uch as a small car). The BenQ SW320 looks interesting, but there seem to be some concerns about uniformity.
You can do all of this and get extra real-estate by extending the moniotors. I quite often have quite a number of windows open that are hard to keep track of on a single monitor. It makes no sense to mirror monitors - you can extend them, and if for some reason that you might be able to rationalize, turn one of them off.
Extending the displays, in the context of gaming, does not affect performance because the graphics card is doing the 3D work for the display you are gaming on. Any 2D stuff in the other display is insignificant in comparison. I sometimes run monitoring software with many updating plots when I am gaming and it does not affect the fps in the games at all. And if you are only doing 2D work, with the power of the graphics card you are considering, 2 displays will both run with ease at low GPU core frequency.
