The more wheels you have, the more even the track pressure at the point of greatest stress... however, every additional wheel/brearing adds a little bit of resistance. Probably negligible though.
Most of the removal of outside wheels is part of WEIGHT reduction, not really for reducing drag. If you look at all the hardcore mountain sleds trying to get rid of every extra ounce, you'll find that they probably have 4 wheels in total -- 2 at the back and 2 on the top.
I personally like a 4-wheel back axle setup. If any one bearing blows, or even 1 from each side, you won't be left stranded. Just as long as you don't lose 2 on the same side, then you're walking.
Wheel size.... don't bother messing with stock. The stock wheel size is a balance between clearance and resistance. You go with a taller wheel, then when you land hard on the back, the track will hit the tunnel. You also have to be concerned about the offset of the shaft from the rails. If you go with a taller wheel, then you MUST go with an offset axle, otherwise the wheel will protrude below the rails and end up creating tons of resistance, hit things hard, and ultimately break prematurely.... and not necessarily the wheel will break. Could break your rails.
More important than the height of the wheels is the material that the outer edge of the wheel is made out of. The harder this material, the less resistance. If you're going to be 100% deep powder, it is OK to go with metal right to the edge. If you like spikes in your track or operate in areas with rocks/dirt/other abrasives, then you compromise by using wheels with rubber.
