I recently managed to send Sandbag flying over 8000 ft. with my attack modifications to Giga Bowser in Brawl's Home-Run Contest. Start out with a d-smash, a few d-tilts, stop the knockback with a single standard attack hit, and then finish up with a charged f-smash with the head, and Sandbag's damage goes over 200%.
Note: While the screenshot is showing Bowser's battle portrait, the Home-Run Contest session was actually done with Giga Bowser, as I never altered Bowser's attack damage in any way.
I can tell you this much though, that without my buffs, Giga Bowser pretty much wouldn't be able to send Sandbag flying very far, since he can't use the Home-Run Bat, and I don't think I have to mention how pathetic the f-smash's knockback statistics are normally.
On a side note, the buffed f-smash sends opponents flying at a 45 degree angle, instead of the Sakurai angle.