Most say stock is the best. Some say it's better with a efficiently designed chargepipe and stock airbox. Others say that's not worth the money. Your best bet is just getting an air filter and calling it a day, really. I got Gruppe M intake plus Dinan Chargepipe, and while I did see some improvements, it probably didn't justify the cost. I'm sticking with N/A and I'm not stroking my motor though, so I'm happy with it.
Wait, I mean, I don't have a Gruppe M intake or a Dinan Chargepipe. My signature says so.