[EricH]

If you do opt for the omnis, you'll want to remove the drop from the center wheel.

The reason why: if you leave the drop in there, you shortened your wheelbase (the point of the drop), while simultaneously making it easier to turn the resulting 4WD (the point of the omnis is to make the 6WD easier to turn). It'll most likely try to act like a 2WD, which is really easy to turn, but might not be the best with traction.

What I might suggest instead would be to:
1) Increase the center drop slightly. This will pick the "up" set of wheels up just a little bit more, resulting in less scrub from them.

OR
2) Change the center wheels to pneumatic wheels. This will allow for slight variations in the drop without permanent modifications, and, depending on the wheels you use, it may increase your center traction. (It may also be fairly simple to test out during the offseason--make your modifications and drop them in.)