you can use either a keyhole saw or a knife. if you know where the studs are you can try to cut right down the center of them with a knife, this gives you a way to put the sheetrock back up easily. you can cut out the whole bay that covers the pipe.
NY State Trooper: So what makes your car so special to have SVT all over it?
Me: Er...It was made by Fords SPECIAL Vehicle Team?