Are you just theoretically speaking or are you referring to the GPU specifications included?
i am considering the benchmarks on this page:http://www.techpowerup.com/reviews/AMD/Catalyst_12.11_Performance/23.html
and then believing that both cards being the same "model" would have the same better performance than a "reference" card.
to be honest, i would not be surprised if the 7770 card you mentioned performs better than the 6850 you listed since it may overclock better with the GCN gpu
. but that would be a little too speculative.