I've been playing a long time at 60hz, the fluidity I get with 120hz is awesome, but would like to get a better image quality. I got it to work at 3600 finally, but its a bit sluggish.Hmm that's a new one, haven't heard of the nvidia control panel freezing like that. I'm assuming you're using the latest-ish drivers?
What you can do is try to decrease the refreshrate to 59Hz, if you've already tried that you can try to slowly test your way by decreasing the amount of "total pixels", for example, the manual settings I am using for 3840x2160 have the values of 2020 pixels horizontal and 1090 pixels vertical. So in other words, you can try 2019, 2018, 2017 etc pixels which results in a lower pixel clock that might yield you a working resolution.
On the subject of benefit it's a matter of preference, you're using a 120Hz monitor which's main attraction is the 120 Hz refresh rate I assume and the input response / smoothness you gain from it. The benefit of downsampling is the gain in image quality and anti aliasing.
I kinda got it to work at 3840, but maybe not quite, when I do the test the screen comes on ok but it kinda goes. What I mean is a correct image shows, but for a few seconds, it goes to black and comes back. Does this mean it is not a good resolution?
Also at 3600 and 3840 I get about a 132Mhz pixel clock, but in 2880 I get a 178Mhz pixel clock, but it seems to work better at that lower resolution, why is that?
I'll keep trying my settings a bit to see what works best. Shoud I lower both horizontal and vertical clocks at the same times always?
Thank for your answer.