Again, you need to check Yosemite about, no performance impact on dpi independent desktop since the new renderer is optimal work to render at 216,300,400 dpi don't need to upscale then downscale.
About you and your retina mbp it's true, it can drive 5 million pixels, but the iMac it's supposed to be capable to drive 2 screens, at 4k are 16m pixels at 5k are 30+m pixels simple no single gpu setup it's capable to do that yet. Add this fact usually iMac 27 users do something more stressful than read email, surf the Web and watch p0rn...
I haven't used Yosemite. Are you saying that it no longer downscales when using a scaled resolution? With Mavericks if you use a 4k display and you want to have an effective resolution of 2560x1440 it renders a 5120x2880 desktop and scales it down to 4k. At least with Mavericks having a 4k display is not less work for the GPU than a 5k display would be.