Being in the car business, I have seem plenty of work come from shaddy body shops and plenty of good work come from good body shops. But seeing its a bumper, you might be better off just replacing it vs. repair. A repair will show from the inside and depending on where the damage is, it may or may not be noticable. Good shops that I have worked with is Desert Collision on W Sahara, Gerber Collision, and Collision Authority ( several locations I think). I do know that Team and Friendly Ford both use Friendly Ford's body shop.
As for any warranty work, once the part is damaged/repaired that part isnt covered under warranty by Ford, it needs to be backed up by the collision center who does the repairs.
PM me if more info is needed.