This doesn't affect retina devices so your 5k iMac will look the same it does now...
Apple is removing sub-pixel anti-aliasing for all devices, not just non-Retina ones.
Here's a possibly better comparison, because it annoyed me that the windows weren't aligned: http://www.framecompare.com/image-compare/screenshotcomparis...
Zoomed in on my 10" iPad Pro the main thing I notice is kerning differences.