Little trick I picked up from another forumer (apologies, I forget who) - if your follower is refusing to take any more loot from you because they're full, you can drop the item(s), put your follower in Command mode, and then instruct them to pick up the items from the floor. This allows you to 'overload' your followers, but it doesn't affect their movement speed or anything.
One thing to watch out for, though, is that if you drop a stack of more than 5 items (so that when you drop it, it asks 'How many?') and then ask your follower to pick up the stack (which spawns as one item), they sometimes will only get one of the item and the rest will vanish.