It's not very clear when you get the pre order bonus IN GAME. I heard with the classic pack that you get it from the doctor at the start, but what about the others? I am getting the mercenary pack, but I don't see that fitting in, I just don't think the doctor would give you a grenade launcher at the very start of the game.
Getting the stuff from the doc (probably instead of the Vault 21 jumpsuit) seems the most likely.
Overpowered though some of it may be, perhaps he'll justify giving the stuff by saying the stuff belongs to you? Maybe the grenade gun doesn't have much ammo or something.