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My guess is that we won't see any new 5K iMacs until next Spring, perhaps late Winter. I think Apple will wait for the appropriate Skylark chip to be ready.
Upgrade to the 21.5 to retina and minor bumps to 27 retina likely in October (if not Sept 30th). Likely better color depth on both. Graphics chip 395x (minor update to 295x) in highest end. Skylake is barely available anywhere in the real world.
 
Not the same as the Retina iPad - the iPad couldn't change the resolution, the iMac can. You can play games at 1080p or 1440p on a 5K iMac, the same resolution you can play them as a non-retina iMac.
I read somewhere that running a game at anything but the iMac's native resolution lessens the game's performance. I don't know if this is actually true or not, and if it does, by how much is lessens the performance, .
 
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