A friend told me if you do this you get some code or some shit that you put in that makes your bullets stronger, thus giving you an advantage online?
Heavily doubt it.
I've pre-ordered the collectors edition (DAT STATUE AND ART) and the multiplayer bonuses with that are classic skins and the "Disorderly Conduct Multiplayer Weapons Pack" (I think an auto-shotgun, molotov everything and a rifle IIRC).
I have seen note of another pre-order pack, but the contents are:
Originally Posted by Wiki Page
Rockstar collaborated with several retail outlets on pre-order bonuses available through several store chains throughout the world. The Cemetery Multiplayer Map is a map pack, where pre-order customers get early access to the map. The Silent Killer Multiplayer Loadout Pack includes the "devastatingly destructive" Light Anti-Tank Weapon, the slippery character burst to quickly escape a deadly firefight, and the listening device item that amplifies hearing, allowing the player to hear approaching enemies from a greater distance. These who pre-orders the game from specified retailers receive a code that allows to get the game's first paid Multiplayer DLC Pack for free.
Will be waiting until I've had a little time with the multiplayer before I decide on buying the DLC season pass.