That's why you don't use a lag/screw. You use appropriately rated toggle bolts, which we've used and then hung our body weight on the bracket ALONG with the TV just for fun. There are web sites where you can calculate what the different thickness of metal studs will hold. Every time one of these threads pops up I hear people warning about how it's dangerous to anchor to a metal wall. It's just not true.

Having said that, there may be very flimsy metal studs out there I'm not aware of (well, they all appear flimsy but you know what I mean). And adding plywood has never hurt.