Guys, he is talking about the hd7870 with Tahiti le (hd7870xt), not with Pitcairn. That card is nearly on par with a hd7950 and can be highly overclocked, a 270x is nearly running at its limit. Only disadvantage is the high power usage.
So at stock clocks similar performance, overclocked the 7870 Tahiti is quite a bit faster