Don't box wintergrasp if the enemy has tenacity, it'll only make them even more powerful.

However, if your side has tenacity (especially high stacks of it), boxing teams will steamroll the enemy (Including vehicles) in short order.

My prot warrior (My main in my guild) was one shotting almost any cloth wearer with 7 stacks of tenacity. We still lost because idiots wouldn't get the hint that they needed to defend the orb and someone managed to get a full click off.