I believe that the loadout has to do entirely with the purpose of the weapon. If the weapon is to be used in competition for example then the modifications would be totally different than if it were to be used for hunting, military or HD scenarios.
For instance in a HD scenario you would probably want a flashlight which, for the M4, automatically means a hand guard with a rail.
In any situation that you choose I believe you build the gun to your specs and then train constantly and consistently.
In some cases that might mean lightning the weapon in other cases that my mean doing some pushups so you arms can easily handle the load.
When I started training with my M4 my shoulder was soar and the weapon seemed heavy. Now I can go through a couple of hundred rounds with no problem and manipulate the weapon easily.