The problem with benchmarks and data is that when you see bars and scores and percentage you often get the wrong impression. I can only give you subjective and anecdotal data - that I ran quite a few games and consistently got similar scores between the two cards. Sure, I ran Fraps while playing and checked the fps to see if there is any difference, but I'd hardly call that scientific. For me, they run the same. It is hard to compare things using benchmarks (although everything else is anecdotal, as I already said) because you can get the wrong impression.
However, I don't have any experience with the M295X so I can't tell you if there are any other differences. The non-retina iMac does run things a bit more smoothly in some apps due to the retina screen, but it's not a significant difference and forcing the 5K to run at 2560x1440 gives the same performance. So, would you call that a tie, or give the edge to the non-retina as it feels smoother, or to the Retina iMac as it feels almost the same but runs a 5K screen? No idea. I guess you'd have to try it before you buy it
One thing I can tell you is that I think the base Retina iMac and high-end non-retina iMac have very similar performance, that I think the screen is worth the small price difference because of that and the base model does not overheat (maximum temperature is around 80 degrees for the GPU and only while gaming)
I've read you have the base retina and you've said that there is no overheating and the performance is as good as the high-end non retina display? If it is like that it is clear that the riMac one would be my first option. I've been reading here many problems dealing with base riMac model so it is very important the information you are giving me now.