DampedOrb :
marko55 :
This could very well all be caused by bad airflow. When that FE card hits 84c it will auto throttle itself to keep cool. That's what is probably causing your bad frame rates.
You have two top mounted fans bringing air in but an exhaust fan in the back right next to one of them which is likely pulling its inbound air right out of the case making one fan useless. Then a 240mm radiator with 2 fans right in front of your gpu sucking air away from it. Those FE cards run hot as it is!
If I were you I'd move the two intake fans to the front of the case and even bump them up to 140mm each. Then get your radiator mounted up top and keep those fans outbound. You can probably leave the rear fan as is. Now you'll have two front fans feeding cool air straight to the GPU and the rest in to the case to flow over your motherboard and up to the radiator fans. Should make a difference.
I've seen simply flipping a rear fan around in a poorly laid out case provide enough additional cool air to a radiator to bring cpu temps down 5-6c before. Air flow layout makes a huge difference.
would it work as well if all I did was change the fans on the radiator to intake and change the fans at the top of the case to outtake?
Actually doing this would be worse. The radiator generates heat. The radiator's fans' job is to blow that heat off & away from the radiator. If you flip those fans they'll be taking heat from the radiator and blowing it as hot air directly on to the GPU.
What you need is some cool air flowing directly at the bottom/rear of that 1080 ti FE where its fan can pick it up, blow it across its PCB/heatsink under its air shroud, and out the back of your case. By having your two intake fans in the front you would be pulling clean/cool air in from the outside and feeding it directly to that GPU.
Alternatively, if you are dead set on not re-arranging your case, you could try one of these:
https://www.amazon.com/Vantec-SP-FC70-BL-Spectrum-System-Adjustable/dp/B000233ZMU/ref=sr_1_1?ie=UTF8&qid=1499950411&sr=8-1&keywords=gpu+fan
https://www.amazon.com/Titan-Adjustable-Dual-Cooler-TTC-SC07TZ/dp/B008A2TDC6/ref=sr_1_3?ie=UTF8&qid=1499950411&sr=8-3&keywords=gpu+fan
Place that directly under your GPU. That should generate some air flow to the GPU. Just check the reviews in regards to how much noise they generate.
OR you could sell your 1080 ti FE and get a 3rd party 1080ti GPU that has a good cooling system from EVGA/MSI/Gigabyte/etc. Most of us that buy the FE cards are only buying them to disassemble them and put a waterblock on them to cool them with a custom liquid loop.