They should be faster(but not by a lot) plus ati has dx11 and it does not get so hot. That's if you put 2 against 2 in dual card configurations.
If you have 2 5870m vs 1 gtx285m there is no comparing them. Also if you compare 2 mobility 5870 vs 1 desktop gtx285 the mobility setup is faster. Your post confused me.