Just found this thread. Funny thing, I'm in the same boat, picked up a 5D Mk2 and right off the bat, I saw a huge difference using flash.
My 7D with 580EX II flash (bounced off the ceiling ) illuminates subject brightly and consistently.
5D with the same flash makes them dim, especially at low apertures (f1.4 - f2.8). Once I get to f4, it gets a bit better, but still not even close to 7D.
One way I found to alleviate this problem is to set flash exposure compensation to +2/3, which unfortunately, doesn't seem like a good solution.
I was using static subjects, so no change in metering.
On a good note, the high ISO performance is pretty amazing with rich colors and creamy background.
I guess, I'll be playing more with it to figure out if I like it enough.
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