You're right about the air gap between the psu shroud and the gpu. That's why I had to get the EVGA ones, coz the 1080ti by EVGA is a 2-slot card. If I were to get a 2.5 slot variants from other brands, the graphics card would be too close to the shroud.
I have been observing the temperatures of my graphics card and I did not observe any massive temperature increases compared to the 1070. The shroud does have air vents so it doesn't choke off the graphics card.
The performance of a 1070 is more than enough to power games at very high or even ultra settings at 1440p resolution and 60hz. I upgraded because I got a friend who has been eyeing my 1070 and he was willing to offer a good price. So I thought, why not upgrade?