Oh nice! very tricky, I now know I am not that crazy and I really saw 60Hz too! Do you have the Nvidia web driver installed?
It should be 60Hz, try moving your mouse around and you will tell for sure. I hope we find a way to have it running in 10.9.4.
I did few test yesterday and even tried the Yosemite Beta 1 that did not even get pass 1024x768 on my second monitor, the dell 4K was dead.
No, OSX default driver.
Mouse cursor moves fast and scrolling in heavy websites is fast too, I don't notice any lag. But League of Legends ran at 33 FPS with sync on, and Ungine Heaven benchmark did not reach even 30 fps at 1920x1080, I suspect that it is running at 30Hz but at the same time it feels like it is more then 30Hz.
The weird thing is that when I tried to play League of Legends in 10.9.4 at 30Hz, it was lagging a lot and I could not move the mouse cursor all the way to the edges of the screen by using the mouse, could move the view properly only by using keyboard. Now when I started LoL and it ran at 33 FPS I could move the mouse properly and there was very little lag. And also, if the monitor was running at 30Hz, how come that LoL runs at 33 FPS, shouldn't it not get past 30 when vsync is on?
If I turn off vsync in LoL and set the cap to 60 FPS, it will stay at 60 FPS (even thought I am not sure if it really runs at 60 FPS) but I can see screen tearing/flickering happening.
It could be that it is really running at 60Hz but there is some performance issues going on which causes games and other thing to not reach 60 fps.
I can also get 60Hz when I install Clover with iMac 12,2 to EFI partition on HDD. No need to use USB to get 60Hz.