y2kpc, I can tell that you're very, very disappointed in your new iMac. I'd encourage not to get so discouraged yet. There may be something to your fears, but I believe that there are other explanations. Be of good cheer, mate.
You're going on the assumption that the Retina 5K iMac has a maximum resolution of 4K or less. I see no clear-cut evidence of that, yet. The reading that you report of "2560 x 1440 (optimal, Retina)" is nonsense. That's the resolution of the NON-retina 27" iMac which is CLEARLY NOT a Retina display on a 27" monitor. This report, which I do understand you merely copied (so not being critical of you), can be nothing but a bug in Yosemite's System Preferences/Display applet. Regarding the "5K wallpaper," the selection box is telling you the resolution of the image, NOT the resolution of the display. I don't think the stepping of the pixel adjustment on icon size has anything at all to do with display resolution. For example, I have two monitors; one is 2560 x 1440 (not an Apple monitor), and the other is 1920 x 1080, but the icon size adjustment occurs in 4-pixel increments on BOTH.
You state further:
"So I am convinced that with the 5K iMac Retina, Apple is using 15.7 million raw pixels (5k) in the "background", but:
1 -- only allows 3.68 Million unique/addressable pixels in OS X (2560x1440/Retina)
2 -- only allows 5.76 Million unique/addressable pixels in OS X (3200x1800)
3 -- only allows 8.29 Million unique/addressable pixels in Windows/Bootcamp (3840x2160)"
The last statement is true because it's a given that there are currently NO Windows display drivers capable of resolution higher than 4K; the first two are simply not true. For one thing, the first two statements are directly contradictory, so by the rules of logic, at least one of them has to be false, but I believe both are. No display was ever stated to be 3200 x 1800; rather, I believe the Display applet states that it "looks like" 3200 x 1800. Is that not correct? And to state [ your #1] that the 27" display on the iMac Retina 5K has the same actual resolution as the non-Retina 27" iMac (2560 x 1440) seems to border on the preposterous. If that's the case, then Apple has opened themselves wide to a large class-action law suit; do you truly think they're that incompetent?
Today, I got to see a Retina 5K at the Apple Store for the first time. It was displaying the Yosemite-logo wallpaper. With my bifocal glasses I scrutinized the details of that photo as closely as I could and compared it to the same wallpaper on a MacBook Pro Retina. I could discern NO difference, and I could not discern individual pixels, meaning that the pixel density of both is Retina-quality -- not identical, but Retina quality. On my 2560 x 1440 monitor on which I'm writing this, I can, indeed, make out individual pixels with the same spectacles. Please look as closely as you can at the Retina display on an iPhone, an iPad, or a MacBook Pro to see if you can make out individual pixels in a gray or white area and then perform the same test on your 5K. Please let us know the results.
Until there's an explanation by Apple engineers or highly-qualified 3rd-party testers, I'm not thinking that Apple "got around the maximum bandwidth limitation" by making the display lower in resolution than they're claiming and advertising. In fact, I'm off to order one. (The Apple Store here stocks only the base model; if one desires more than 8GB of RAM or the R295 display adapter, it's necessary to order online.)