The “back door” for groups, if you go thru it, takes you down a long flight of stairs, then down a hallway, past a gift and souvenir shop, then farther down the hallway, where you exit outdoors, close to the St. Peter’s entrance doors, on the “OK, cleared” side of security. It was surprising seeing the long line of people waiting to be cleared for entry to the Basilica, while we walked right in.
Anyone backtracking to the Vatican Museums entrance has a long walk, then they have to go a long way to get to the St. Peter’s entrance. Without the “back door” exit from the Sistine Chapel, you’ll wind up doing that distance 3 times, once to get to the Sistine Chapel, a second time to retrace to the Museums entrance, and then again to get to St. Peter’s.