I... actually like that solution. But, another solution I've been preaching for moths is: Make the damage a weapon does proportionate to its versatility.
What this means is AV grenades are simply too versatile to be this powerful. In most cases, they eliminate any incentive to specialize in AV weaponry that require a weapon slot. In fact, they take jobs away from AV players. See a tank on a field? Merc A makes a beeline for a supply depot to change to his proto swarm launcher fitting before the tank blows it up. By the time he changes fittings and catches up with the tank, merc B is already lobbing AV grenades at it while camping a nanohive.
AV grenades are dealing the damage that real AV weapons should be doing and their users don't even have to change fittings. Speccing into a real AV weapon should reward you for doing so because you have meaningful choices as an effective counter to a specific game mechanic that other players, at the cost of versatility, should only have a fraction of that effectiveness. It's common sense.