Wow, you guys are great! Thanks so much for your help that I'm just now getting a chance to thank you for.
Just a few comments because this has not been resolved on a technological level. I ended up selling the XPS 27 One at a nice premium, so none of this matters anymore on a financial level. Still, I want to share my first-time experience with any Dell product and Dell support.
It seems to be that only the very senior support staff has any clue beyond answering calls requesting knowledge as to turn the darn thing on. (yes, exaggerating, but only a little) In fact, right before I sold the XPS 27 One, the next-generation Haswell refresh brings a new XPS One 27 with guess what, a Thunderbolt port!!! Which, of course, doubles as a Mini Displayport, and any form of Displayport has NO problem with 2560x1440! That right there would have solved my issue. So I call and keep getting transferred around and one technician, after giving up a little secret, which was that Dell itself capped the HDMI IN port at 1080p "with its own proprietary drivers". "Wow.", I thought. So I asked (well, begged is more accurate) if we could somehow swap it out and I would pay some kind of fee as well as shipping both ways, even though I had bought the XPS One 27 only a WEEK before...but not from Dell, which is why I asked that.
Simply declining my request was what I thought would happen but no, the Dell rep. let me know just how little they care about their customers when he said: "Well, why would we do that?" (exchanging my week-old system for the JUST-released one). "Because you care about your customers???" I yelled into the phone in disbelief.
Thunderbolt would have been SO nice, too, as I use Macs these days, but I used to be a ThinkPad fan, so I'm not at all anti-PC. So I wanted to dabble in Windows 8 and the whole touch thing, but boy was I disappointed.