I spent some time today dry firing and taking video. I have to say that with a 5 pound weight (about 8.75 total CW) falling 7 feet, I only got a small portion of the energy into my light weight 2 foot arm. If built correctly with inelastic parts, 100% could be possible, so clearly there is room for improvement.
I have some ideas to help; I'm pretty sure I can get at least half of the energy by modifying some parts, though I will likely have to remake the complex trigger mechanism.
For now, I'm going to fix the few issues (slipping knots and such) then go throw some stuff. In the bright side, this machine could dry fire safely loaded with 50 pounds. A miss fire where the sling actually deploys could be an issue though.