I just came back from a local Bestbuy and HD4000 didn't look "fine" for 2880x1800 (or 2560x1600, didn't check). It played movies alright and scrolling paged documents looked smooth. But as soon as I started something real-time (rendering), things got slowed down quite visibly. Simply resizing windows looked sluggish. To Apple's credit, it wasn't the kind of stutters that you usually see in Windows when a GPU is running out steam. It was more of a decelerated/slow-mo'ed animation, rather than stutters. That definitely helps reducing the irritation factor but still, I found myself getting impatient regardless.