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I have the high end 21.5. It does well on high at native res - starts out near 60 fps and gets into the 30s in big battles. Never seems to go lower than that (fwiw, ultra at native drops into the teens in big battles - high is the way to go on my model).
I think the 6770m is 10-20% faster than the 6750m, so one might guess you'd see fps drop into the mid-high 20s on the base model iMac in big battles on high at native res.
As compared to your mbp, it's the same card but a higher native resolution and half the VRAM, so you'd probably see somewhat decreased performance.