Thought about adding weight like you did, but I was afraid of having it sit too low and having to adjust the torsion bars anyways, so tried this instead. From just running it tonight, it does make the rear end a little stiffer. As an added bonus, it tightened the track tension which I figured it would. Still have to test it over rough terrain.

One thing I am worried about is either snapping the two torsion bars, or worse, snapping the whole attachment points off of the hull from the added torque on them.