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anandtech does comparison on the older cards but thats about it, mayby tomshardware will have a few graphs etc
http://www.anandtech.com/bench/GPU11/188

Very few sites do a proper indepth look at multi GPU performance unlike single cards there are a lot more variables as to how well they perform - the recorded frames per second can be a lot less accurate in how well they actually perform than with single cards due to things like "microstutter".