Here is my experience from gaming on the iMac with Retina. I don't have a Mac Pro so can't tell you about that.
I have the upgraded CPU and Video Card and put in my own RAM. I made a small Boot Camp partition and have an external SSD in a USB 3 case that I keep my games on. You can't get more than 4K out of Windows, so 5K gaming doesn't really exist. However, at 4K you won't get 60fps out of any serious games. I run both my desktop and my games at 1440 and they look terrific.
My previous rig was a custom PC with a GTX 760 card. The iMac is faster than that but not hugely different, maybe 10% or 15%. As far as a gaming machine it is quite capable. At home I have Unigine Heaven benchmark if you want that. But usually I run games at very high and a few at ultra settings. Usually I just fiddle with settings until I get right around 60fps.
I have had no issues with it other than I worry a little about the strain I put on the video card. The fan definitely kicks in when gaming. Never had an issue, but I wonder if I am shortening the life.
If you have any specific questions from someone who is a serious gamer using an iMac, hit me back, either here or in a PM. From my perspective the iMac with Retina is the most awesome computer in the world and for any current games a more than adequate game rig, even at serious settings.
Thank you so much for this, I was getting quite frustrated but this is the exact info I was after. As mentioned previously I don't expect 5k gaming but 1440 on high or ultra would be a dream.
I may well pm you too, but I am currently on the road for a few months so won't be purchasing until I'm back home, if you play dayz, or do give it a try anytime soon, it would really make my day hearing about your experiences with it.
Thanks again.