Power armour with all the strength related mods (calibrated shocks on both legs, motion-assisted torso and all pieces painted with military paint) allows companions to carry more than the extra you get from the Lone Wanderer perk. With Cait, even easier than the pick-lock-relock-with-terminal-quicksave-load-save method is stripping the player character naked and fast traveling between two reasonably distant locations (I did red rocket and hangman's alley, she loved it).