The more of a shield the case forms, the worse the performance will be. If the case covers the entire back and sides (except for the required holes for earphone, pen, etc.), performance will probably be terrible unless you're in some place like midtown NYC, where there's a cell antenna every few feet. (Where I am, the tower is about 5 miles away, and I can drive another 15 miles before I start picking up the next tower. With an aluminum case I'd barely get a signal here, and I'd lose it within a mile of heading toward the next tower. (Pretty much the same physical arrangement - a Note 3 - so probably about the same performance.)
Besides, aluminum doesn't absorb much shock, so if you drop the phone in an aluminum case, a lot of the shock will be transmitted to the screen and it could crack. TPU and rubber cases help to prevent that. (I've dropped mine pretty badly a couple of times, in an Otterbox Defender. It still works.)