Lowering the firebox after getting rid of the hearth will mean changing the guts of the fireplace, which will mean work on the flue, replacing the floor, etc. However, as it now stands, your TV would need to be pretty high up if you want to keep the raised fireplace as well as have a mantel. A compromise would be to keep the hearth to avoid all the work of redoing the firebox, and leaving off the mantel so the TV could hang bit lower.