Well, I'm using the new 2017 5k iMac with i5 and radeon pro 580. I'm just a casual gamer, but I know what I would get with a high end gaming pc. My last one was i7 7700k and 1080ti.
I just could not give up the 5k screen and whats more important, MacOS. If this matters to you, you have no alternatives.
For gaming I bought an external USB 3.1 SSD (Samsung t3), installed Windows 10 and there we go. I can play anything on high and ultra with a resolution of 1440p, the 5k screen scales superb with 1440p. Temperature in AAA games are around 85°C on CPU and GPU. Fan is not audible on desktop use, in games you can clearly hear the air (wushhh) but nothing like a low quality fan noise.
So much less cables!