So I just received my 5K 2017 iMac
Quickly ran Unigine Valley
Ultra 2560x1440 w/ AA-Off
iMac 5K 2017
4.2 GHz Core i7
16 GB RAM
Radeon Pro 580
Default Fan
GPU 80ºC
Score 1696 - AVG FPS 40.5
Max Fan
GPU 72ºC
Score 1705 - AVG FPS 40.8
Compared to my old iMac:
iMac 2013
3.5 GHz Core i7
16 GB RAM
GeForce GTX 775M 2GB
Default Fan
GPU 95ºC
Score 1229 - AVG FPS 29.4
Max Fan
GPU 92ºC
Score 1229 - AVG FPS 29.4
Performance is quite a bit better and temperatures are much better, I'm not sure I'll do more benchmarking than this but I am quite pleased to see the GPU stay cooler during operation than the previous iMac (with its supposedly much vaunted GeForce Graphics).