Even though I don't like what you said (as in: it doesn't ease my mind): Thank you for describing it in so much detail!
Apple didn't call me back yet, so I don't really know if they were able to do sth. about it or not. When I'm going there to pick it up I'm going to take a friend of mine with me for the sake of the psychological effect - so the Genius Bar reps / the manager can't turn me down and tell me they don't see anything.
If they keep saying the stuff we all heard multiple times, a few things will happen (and I will make this crystal clear to them): I will check my options to place guest articles on several IT news sites about the issue itself and the absolutely ridiculous way Apple is handling it - in fact, writing these mails is what I'm doing right after this. Plus I'll get in touch with consumer protection in my country since the product is not as advertised. And still call Apple Care to climb up the food chain a bit. The one thing Apple really cares about is its image which makes it their weak spot as well.
Remember: as long as we're discussing here, it stays here. People who don't know about this problem will not search for it and when they got their own device they may or may not see the lag. At the Genius Bar they can turn us down. But if we take it out to small but well known sites this might force Apple to acknowledge the issue. I'm not saying that it will have any or a huge impact as I can't promise this, but at least I want to put up a good fight here. If they behave like a government official, it's fair to treat them this way: dig out the dirt, make it public and hope for it to get viral.
@Krusty: Apple Hardware Test shows the same for my device. But most of all it's a quick hardware sanity check, it doesn't do any in depth checking as far as I know. Yet I believe they can't even give us a solution except all new all shiny hardware. I've read an article on AnandTech the other day where they describe how they do they scaling on the rMBPs. While in "best for retina" mode the image is rendered at 2880x1800 (or w/e it is exactly), on scaled modes like the 1920x1080 one I tend to use, the HD4000 has to push 3840x2400 images. In the best case at 30+ FPS. That said, the HD4000 isn't even capable of doing the 2880x1800 resolution in a fluent way at all times. Plus because of the way some applications (I.e. browsers w/o hardware acceleration) work this puts a lot of strain on the CPU as well, which is no where as good at drawing UI elements. AnandTech states that in programs with high quality assets (like InDesign for me), "the CPU has to decode images at 4x the resolution it is used to."
All this brings me to a simple point: they build a laptop which they advertised in my country with 'Power bis zum letzten Pixel' (powerful to the last pixel) but simply cannot deliver. The hardware they use is nowhere as powerful as it should be to drive this resolution efficiently. Which explains why the late 2013 models have less problems, as the Iris Pro is way more powerful as the HD4000, but still lacks performance when it comes to high resolutions. Even the GT650M isn't powerful enough to drive these resolutions (exc. "best for retina"), the new one might be.
Here's the link to the whole article: http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-r eview/8
You may agree or disagree with the above, but it's the best explanation I've come up with so far.