Mounting TV on metal studs?
If you do not want to remove drywall to add wood structural support to the inside of the wall you could take a piece of cabinet grade 3/4 plywood and attach it to to the wall so it spans two studs and is bolted to the the metal studs, stain finish it or paint it to match the wall. You could use dimensional lumber as well if you are not as picky about appearance. 

Cut it wide enough to span the studs, 28". Attach it to the studs with toggle anchors, at least 4, one at each corner, but 3 per side would be better. Then you can use heavy duty toggles to secure the mount to the wood.