The Thall and AR7 are both not symmetrical skis, and thus will not ski the best at true center. You WILL be fighting the sidecut of the ski if mounted there. My own AR6's are mounted at center, I fucked up.
It depends on where you're riding, but I know I talked to Tyson Hall when my bro was picking up his Alpha 2s last season, and the farthest FORWARD anyone mounts their own bindings on the Armada team is 1.5 forward (Tanner himself).
Mount them at 1.5 back for park. 2.5 back for all mountain. Pay attention to what the makers of the product are trying to tell you, they want you to be happy