This 6-wheeled omni drive setup is exactly what Team 343 did in 2002.
Here is a picture of their robot. I can see that they used the Fisher-Price motors and gearboxes to drive their "sideways" omnis. I don't know what motors they used for their "forward" omnis. This robot won at least one Regional Competition, and was extremely effective at moving TWO 180lb goals around the field. So, I think that would probably put to rest the question of "how much traction" can you get from this setup.
This robot has a very special place in the students and mentors of team 343. I'm sure any of them could tell you more about it.
BEN