The GTX1080 Founders exhausts it's heat through the expansion bracket. You shouldn't have any problem felling that if you put your hand up to the vents near the display outputs on the card while it's under load.
Aside from that, the Ryzen 2700 is really the only thing that's exhausting heat into the case, and it doesn't draw a terrible amount of power. About 100W max. However, as I said before, since the case is so perforated, that heat is being exhausted out of many different areas. If you had a forced exhaust (aka install the rear 80mm fans), you'd probably be able to feel a slight temp since they'd be attracting a fair amount of heat, and the other open vented areas wouldn't get quite so much heat blowing out of them. I don't think it's necessary to install the 80mm fan(s) though. The other factor is that you've got a lot of CFM into and out of the case. Imagine I've got 100 rubber ducks (Watts) and i float them down a small stream (low airflow). I can stand in the stream and catch quite a few of them (feel the heat on hand). If they're floating down a large river (mucho airflow), I probably won't catch many at all. Make sense?
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