Terminal velocity must be pretty ding-dang fast!
200 feet of stone is tough.
Note I said 200. Fordam is 300 feet.
You still have to hit the same hole twice.
Kinetic energy is the key to its success. The GBU-57 is so darned big because of the depth it must penetrate to allow a conventional explosive to damage the target.
While Ground Coupling works with conventional as well as nuclear it just isn't as great as the nukes, thus getting it deeper is a must.