What are the temps you're alluding too? I would just plug it to the motherboard and run the rad fan anywhere from 1000-1500 RPM. Being it's only a 120mm, I bet it's getting heat saturated and not enough air flow to it. Find a happy medium and leave it at a fixed speed.
So on the first night after I installed it, playing Battlefield 4 at 1440p 144fps Ultra settings, I was getting a consistent framerate, but getting well above 70 degrees Celsius. Modern Warfare at 1440p with medium settings or lower was also running smoothly at 144fps, but also getting pretty warm. Playing the Minecraft RTX beta really pushed the card, hovering more around 90 FPS and maxing out the card. One or two of those three games were regularly hitting the temp limit of 84 at times. I don't have enough knowledge or experience to tell if that's normal or not, but for a watercooled card that I'm not even overclocking, that seems excessive.
Earlier today while I was trying to get some more specific notes, before I noticed and diagnosed that the fan simply wasn't working correctly, playing Killing Floor 2 (a game my 980 could run at 1080p 60fps on Ultra settings with rock-solid performance and ~55 Celsius temperatures) at 1440p 144fps on High settings, I was getting 60-64 degrees, but that was after many minutes of gameplay as the temperature slowly built up to that peak area. Out of curiosity, I played another game with the same in-game conditions and settings, but took the glass side panel of my case off, and was getting 55-58 degrees. Halfway through a game, I put the panel back on, and the temperature climbed back up to the 60-62 range.
When I turn my PC on, my GPU starts at 30 degrees Celsius, which is a little warmer than room temperature and 10 degrees lower than my 980 was idling. But slowly it builds up to 40 degrees and idles there, which seems odd that it wouldn't stabilize sooner. Like I said, gaming is the same way, with temperatures climbing and descending very slowly and gradually, far more slowly than my 980 Classified ever did, and far slower than my CPU which has a 240mm EVGA radiator on it with the same fans.
My case has very good airflow, and the air coming out of my top two exhaust fans is almost always cool, as was my rear exhaust with my 980 before I put in the 2080 with its radiator. Now, the back top exhaust (right above the radiator) is often a little warm when gaming, and the rear exhaust is very warm. The top fan farther away from the radiator is still cool.
Besides the logistics of unscrewing the fan and radiator and doing some cable work to plug the fan into the motherboard, my concern is that unplugging the fan and using the motherboard would mean I can't have the fan automatically adjust speed with the temperature of the GPU. If I had to plug it into the motherboard and run it at a higher, unchanging speed, I wouldn't lose any sleep, and I bought these Noctua fans specifically for their performance and quietness, so it wouldn't be an issue. However, I just want to know if there are any other alternatives that might get it to work. EDIT:
I opened up my computer, unplugged the fan from the 2080, plugged it into the motherboard instead, set it to run at 10 volts, which is about 1250 RPM, and now my temperatures seem much more normal. KF2 on the same settings as earlier ran at 50-55 degrees, so that's a 10 degree improvement and seems much more reasonable for what I'm pushing. It doesn't slowly build or decrease in heat like it did before, either. So that solves my temperature problem. ADDENDUM QUESTION:
Should I consider getting a second fan to run a push/pull setup? How much would that improve my temperatures? I'd like to push the card higher, as in running higher graphics settings and still maintain cooler temperatures/consistent performance, and adding another fan is doable for my setup.