Chris Hibner wrote a great paper on 4wd turning.
http://www.chiefdelphi.com/media/papers/1443
I would recommend reading through it. Basically, a 6wd drop center is essentially 2 4wds (as others have stated).
An interesting phenomenon can occur with 6wd with a reasonably flexible chassis. Even with stiff rails, if the two sides are not torsionally stiff relative to each other, then diagonal outer wheels can end up touching at the same time. This will cause it to act like a longer wheel base which can reduce maneuverability.