Mounting articulating Arm for TV (43”) to wall with metal studs
An articulating arm swings away from the wall and this creates different forces than just attaching directly to the wall. You didn't note the distance that it can articulate nor the weight of the TV nor the vertical distance between the bolts of the bracket to the wall and all are factors that determine the 'moment' and therefore the force of the tension / compression affect at the wall (consider using your hands to hold a piece of plywood up vertically by the edges, you'd have to push out at the bottom and pull in at the top, as well as lift up the weight of the plywood to keep it in place).

For metal studs the critical element is most likely the flange or face that the dry wall and bracket are attached to, it will bend out of plane, in and out of the wall, likely before the stud as a whole would fail. What resists that is the thickness of the metal. The force required to bend the flange likely isn't high. Metal stud design is COMPLECTED and there are MANY different types (some are very flimsy) and of those the least expensive is chosen, that is, the one with just enough strength to do the purpose. 

With that in consideration, and that you have access to the back of the wall then I suggest attaching directly to new full height wood studs. You can easily locate the studs by first determine where the bracket needs to be located based on where you want your TV, drill holes through the dry wall as if you were going to attach it, but don't yet of course. On the back side, use those holes to located the new studs.

Strong and easy peasy.