Is it possible to alter the speed the Low/Med/High provides?

As far as I know, only the "maxSpeed" property of workcarts can be easily adjusted, and Hi/Med/Lo simply correspond to fractions of that value (1.0, 0.5, 0.2), which means you can't easily customize the speeds individually (without incurring a significant performance cost or making it really complicated). There's also a property called "engineForce" which I think just determines the acceleration but I haven't tested this in a while.