I think if you really want to or need to pack heavy, there is nothing like the MEI Voyageur or Executive Overnighter bags. Real backstraps, with sternum straps and waistbands. You could carry 20kg with that.

The waistband (or do you say waist strap?) takes the load of your shoulders and moves it to the hips, if it fits correctly. The sternum strap makes the bag fit closer to your body and thus lighter and more comfortable to carry.

Till