applekev, I too am a southpaw and I too own a few ar-15s. I like the AR-15 BECAUSE IT IS PERFECTLY AMBIDEXTROUS OUT OF THE BOX.
I shoot from prone, bench and standing with my ar-15. I find absolutely no problem with shell casings or anything of that nature. I really don't even know what a shell deflector is, but if it's got the ability to put your brass in a neat pile, I'll take 3 of them, one for each AR I own. I have, however, shot a bull pup design ar-15 and found it to be horrible for lefties.
I'd be careful about shooting a bolt action rifle left handed, but the AR is perfect for lefties.