What is "won't make much of a difference"? 5% more airflow? Could be a big deal in a tightly packed case like this.
To me, the lines on the rectangular vents work with rectangular shape of the case more than the round holes.
R290's no longer use the crossfire bridge, they use the PCIE bus to talk to each other. So there is a considerable jump in bandwidth need compared to previous cards. edit: I see you already know this now. H's R290 crossfire review said 2 cards use 750W so your 850W should be enough.
The daylight shot isn't that bad, that pic has too much jpg compression. The art style doesn't rely on excessive contrast and saturation. Plus it's PS4 footage, so it lacks fancy AO.