So... I had another (semi-silly) idea.
Why not use a sort of counterbalance in the left hand? namely, the 30 pound top of a plastic 55gal water container, which is about an inch thick at its thinnest point, and nearly 2.5 at its thickest.
It seemed to help a tad with distance, but as I did not measure my shots, I cannot certianly say it did. I was slinging snowballs.
They only fly about 50-70 yards to start. I was probably pushing 85 with the shield. I honestly don't know why this happened. Chances are it has nothing to do with the weight of the shield. Here is the reason I thought it might-
http://www.youtube.com/watch?v=PAGpO4AA8g4you can see arm barely moving, while his body sort of levers off of it, with the sling arm moving VERY fast. even in slo-mo.