I have a 30" Dell Ultrasharp running at 2560x1600 connected to the 27" iMac and see no screen redraw (or any other) issues at all. Surprising, because I kind of expected Photoshop to run faster with the external monitor disconnected, but I don't notice any difference at all.
I don't know what you use your Mac for (business), but unless you're running Final Cut X, you're likely not going to see a whole lot of bang for your buck for quite some time (until all the software companies re-tool their software to take advantage of the MP's new stuff).
If we're talking benchmarks, then yeah, the MacPro is going to show an increase in speed for various tasks (though not all). But for everyday use with web, email, office apps, and even Adobe InDesign, Photoshop, Illustrator, Acrobat, etc., I could not see any noticeable speed increase.*
I DID notice a slight increase in speed copying large amounts of small files from a server compared to the iMac, but again it was hardly worth noticing. YMMV.
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