Thanks guys!
I got a hold of some ACME thread today and made up a new shaft for one of the screwdrives. I was hoping to increase the speed of the dump without having to invest in different gear motors and it looks to be a success. It's still not really fast but is noticeably quicker and the motor seems to have enough torque to handle the increase in ratio. Here's a short bench test with 2 lead acid batteries in the front of the box.
http://youtu.be/Yff0H9PKLuY