I've heard of them and ghost shrimp eating pond snails, but not the ramshorns as much. I have found that my cherry shrimp seem to stop all reproduction of snails in my tanks. I have not observed to see if they eat the snails or the eggs though, or even if it is just a matter of limited food supply. I do know that they don't attack the large adult snails as they continue to live and grow in the shrimp tank.
dave