Canon MK II is a champ at f/2.8 resolution, look at that center frame f/2.8 performance on an optical bench! Even for MTF80 it is still near 0.8 while Nikon is down to under 0.65 and Tamron 0.55. The Mk II is capable of really driving high-density sensors super well wide open.
Of course it also shows, that as people say, a body with more MP + a lesser lens always outdoes a body with less MP but the most amazing lens when it comes to total detail. So D800 + either option captures more detail than 5D3+MkII (as expected). The D800 could use an even cheaper lens and still deliver the same total detail. Which again goes to show all the talk about high MP needing the best lenses to work well compared to lower MP sensors doesn't (and has never begun to) make sense (although it does make sense if the goal is to get every last bit of performance out of them, which a MkII seems it could do better than the other options).
But the MkII should still rock really nice micro-contrast center frame on a 7D at f/2.8 while the Tamron probably won't resolve the tiniest finest details quite as well.