My GTX 1080 SC is running at 80° during games and benchmarks, the Fan is at around 48%, 1440RPM.
No overclocking, no custom fan curve. Just stock as it came.
Shouldn't the card be cooler?
Driver 368.39, BIOS v86.04.17.00.80

Try setting a custom fan curve to lower the temps.

These cards are designed to get that hot.  You really should worry if the temp starts exceeding 90c and even then these chips are able to run up to 100c.  But seriously, if your card were to start getting hotter than 80c, your fans would kick up to 100% before it let the GPU temps get hotter.

Try something like this: 
 

 
 

Ok, took a quick look at the Define R5 and here are the fan options.
 
Front: 2 – 120/140 mm fans (included is 1 Fractal Design Dynamic GP14 fan, 1000 RPM speed)
Rear: 1 – 120/140 mm fan (included is 1 Fractal Design Dynamic GP14 fan, 1000 RPM speed)
Top: 3 - 120/140 mm fan (not included)
Bottom: 2 – 120/140 mm fan (not included)
Side: 1 – 120/140 mm fan (not included)
 
I am guessing you are using only the 2 fans that came with your case? Add another 140 mm in the front and atleast 1 140 mm in the top (Heat rises). that should create much more air flow thru your case and should drop your temps. (2 intake, 2 exhaust) If you want even more flow you could also add a 140 mm in the bottom and a second 140 mm in the top (Total of 3 intake and 3 exhaust). Since the R5 is more of a quiet design I think your current front fan has much lower intake air flow then a standard case so I think once you bump up the flow your temps will drop. As mentioned make sure you have good cable management as well.
 
 
PS I like using 140 mm fans when able, they move a lot of air and they spin slower (generally) in comparison to the more common 120 mm so that should help keep the sound down for you.

jfolds
How about like a true pc master race member and run 100% all the time. That's what I do.

Is that for folding applications which run the system maxed out continuously?
 

fubu1080
Okay, I did some tests.
Root cause is my case (Fractal Design Define R5)
When I open the case, I get nice 75°, 42%, 1280 RPM (no custom curve, just stock)
 
With closed case I achieve the folowing:
aggressive (too loud for my liking):
66°, 82%, 2450 RPM
custom (to achieve 75°):
75°, 57%, 1700 RPM
 
I assume I should get an additional fan for the case. What would be better, an additional front fan or a side fan? (The case already has one front and one back fan)

I have the same case. I would certainly advice to add another intake fan at the front (fractal design dynamic GP-14). I have my radiator sitting at the top of my case with two 140mm on exhaust and one 140mm fan in the back. I run two intake fans at the front and have one laying around which I am considering to use at the bottom.