For one, don't use furmark. Use another benchmark program that puts a more realistic load on the GPU (3DMark, Valley, etc.)

Second, everyone's PC is different due to ambient room temperatures, case airflow, and even custom fan curves. 80C is warm, but not overheating. Take the side of your case off - if the GPU temperature drops, your case airflow could be better. If not, how hot is your ambient room temperature?

My Setup is an x370 Taichi/Ryzen 3700x/1080ti ftw3/be quiet Straight Power 11 650 watt/2x 1tb Samsung 970 evo plus/4x8gb Gskill Trident z 3200mhz cl14/aro-m14o aircooler. 5x be quiet shadow wings 2 140mm and 1x be quiet shadow wings 120mm put rear. The two 140mm in the front pump air in the case, two blow from beneeth the grafic card upwards to cool it better, one 140mm is on top blow hot air out of case and the 120mm rear blows also out of case.
 
The last one(aro-m14) cooles this cpu at about 35-67° from idle to load and it exhaust the air out of the case, yes. 
 
The temperatures appear also when im playing on 1440p, not only in furmark.
For example, I played star wars jfo on max. details and the card got very hot with loud fans. I had to cut dps to 60fps, but after a while of playing(10-15 minutes) the card got also on 80 degrees and the memory tempe was about 83-84°. The ambient temp. was cool 22° i played on rainy day with half open window. 
I have to note, my first 1080ti had temperatures like ~70-72° under load with less fan spinning. I can't tell if my card gets to hot or not, because i have not enough information of other 1080ti ftw3 owners.
Memory temps @ 84° sounds high from my pov. Maybe i have to go watercooling this card any time^^.
 
Greetz
 
 

I don't have Furmark but I have Valley. And I only have 3 case fans, two at the front blowing in and one at the back blowing out. However, I water-cool my CPU via EVGA CLC 240. I have two 1080Ti in SLI. Both of my GPUs never get near 80. But then again, I customized the fan curve. Perhaps, you should try that. I played COD WW2 multiplayer, Sekiro and Dying Light. Strangely enough, the GPUs fans went up when I was changing the setup of my soldier in COD WW2. If I remember correctly, its temp went up to 69. Them two, mostly hovering around 50 to 55 degree.
You didn't mention where your PC is located. I have mine placed under my desk which is back up to the wall. I have cabinets at both side of the desk. In effect, it is quite enclosed. I have noticed that when the game got a bit hectic, I could feel the warmth coming up from under the desk. What I did was to put a 12" fan under my desk and positioned it to blow at the top of my case as my radiator is at the top. This is to help to move the hot air away so that it wouldn't increase the ambient temperature around the case, well, at least reduce it. I also have a thermometer placed on my desk. Just by having a fan under the desk, helps to reduce the ambient temp down by 2 to 5 degrees. Those few degrees helped to reduce stuttering as well, well, at least in my case. 
 
PS: I read somewhere in this Forum that Precision X1 is for RTX and Precision XOC is for GTX. I have Win 7 with which Precison X1 doesn't work. And there are quite a number of bugs with XOC. I actually use Afterburner to manage my two 1080Ti. Hope this help.

NikeTheBright
So i tested the system now with open sidepanel. For better comparison again with furmark, same settings^^.
 
Idk how i can insert picture, but the gpu temp. drops at about 6-7 degrees after 15 minutes and the memory temp. about 5 degrees. 
And the fans now rotating only with 50%, which is great again.^^ 
 
So I have to improve airflow of the case I guess. I have an old 280 AIO, could take this instead of that giant aircooler, maybe this will decrease temperature aswell.(Have to buy thinner fans that it can fit in front)
I also have the power cable of the card going beneeth the card so it could also block the airflow, i'll put them above the card and monitor the temperatures again.
 
Alexoferith said, that he almost never gets over 69 degrees, I wonder how fast his fans are spinning, when he's gaming. Mostly about 50-55 degrees°, this would be awesome...
I play now some Witcher and monitor the temperatures again.
 
 
Greetz
 

Okay, why would you remove an air cooler to only replace it with creating the same problem by using a CPU AIO as an intake with all that warm air, being used to 'cool' your GPU? You need to exhaust the heat of the CPU, out of the case. Put the CPU AIO as exhaust on the top of your case. Keep the front as purely for intake air and the rear fan for exhaust is your best route IMO.

*Update
 
First @HeavyHami, I only have an 280mm radiator, so it could only be mounted in the front of the case, with the fans of course blowing out of it, and the rear and the top fan now pull the air in the case.
Temperatures were better in the beginning, but after a while i noticed, that the coolant temp of my AIO (EVGA clc 280) went over 50 degree's and still increasing i stopped the test. I have to admit that EVGA software is really bad.
 
I'll go back to the aircooler now and will live with higher Temps and especially higher fan rpm's i guess, because this AIO ist louder then the card can be with this ****e fans^^.
I now let the front two fans pull a bit more air and silence the fans in the rear and top, to hopefully get the hot air better out of the case.
 
Greetz

NikeTheBright
* Update 2
 
I removed the AIO now, put on again the aircooler, removed the 2 fans from beneeth the card, higher the rpm of the 2 fans in the front, higher a bit the 1 fan rear and 1 fan on top, and set the fan of the aircooler also higher so its still quiet enough in idle but also blows away the air better now. 
 
Temps on card mem2 are 4° cooler and card about 5° cooler. Rpm from card fans are from 54%-60%, thats also an improvement at about 10%.
When im playing games, I guess i have to higher the speed of aircooler above to improve the temps in case. 
But still i think i have a hot card in here^^.
 
 
Update 3 
 
I unvervolted the grafics card now stable, and it's running now at 70° on 1974 Mhz thats awesome now with 45% fan speed omg its quiet now.
Undervolting ftw3 :P
 
Greetz
 
 
 

Bah.. I see that on your case you can fit a 360 rad in front but only a 240 on top. I should have looked that up first.  That's too bad. I also undervolt using the voltage curve but at 2050mhz @ 1.025v

Welcome to the club.i am on my 3rd 1080 ti.my warranty expires in less than 2 months.these cards were plauged from the start

same here