Is it sufficient to ground to a box using the NM clamp?
No, that's a "cowboy move" - there are 10-32 threaded holes in virtually every metal box specifically for a grounding screw, and using the cable clamp as a ground connection is not compliant/listed/correct.

The ground wires should all be connected to each other and to that grounding screw. In the unlikely event that your boxes don't have a pre-threaded hole there are thread-forming grounding screws available, but generally you just need to grab a bag of short 10-32 screws (you can find green ones in the electrical aisle, but they don't have to be green.) They specifically need to be the "-32" or fine thread variant so that they have sufficient threads in the box metal - and really, if you look carefully, you will almost always find that hole in the box ready for it.