I have one of the ole Vortex impeller jobbies in my hopper. Basically what I've noticed is that:
* As others have pointed out, the last couple of balls dont get flicked about.
* The impeller doesnt touch the edges of my hopper so it appears to me that it spins faster due to less friction (but could also be the X-board)
* The arms 'capture the balls', and the top of the impeller is angled to help balls roll down into the arm gaps... this helps them feed down the neck better.
Overall buy one, they do the biz, esp if you have an X-board too
Cheers
Ray