@Parker you would be correct, as adding the chain drive would only serve to add friction or weight. (2:1 is a speed ratio but that's not what this thread is) Essentially, you would be going about the same thing in two different ways. One thing to point out is that a 6 motor turbo and 4 motor high speed have the same amount of power, as in pushing power. The math works out (assuming all motors have the same amount of power) 4 motors at a 1.6:1 power. This results in a torque of 4/1.6= 2.5 motors of pushing power. Again, at 6 motors at a 2:1 ratio (or turbo which is the same) it means you end up with 3 motors of power. However slight that difference is becomes negated with the increased stalling associated with going faster. This principle essentially states that when you move faster, your motors have to do more work to slow down, due to the increase in momentum, leading to stalling at about the same time.
My recommendation is that you use a 6 motor high-speed drive, or an 8 motor turbo drive, as has been standard in most of the reveals this season. We experimented with A 6 motor turbo and found it stalled too often and was too inconsistent with stalling and lack of torque. So that's my recommendation, I've seen robots pull off a 6 motor turbo, however, this year with the cones I don't recommend it.
Thanks for the helpful advice, I was slightly worried about turbo anyway. However, that still leaves the question of a 6-wheel direct drive base vs a 4-wheel chained base. I'm assuming that since you agreed about chain adding extra weight and friction that you'd recommend the 6-wheel base, but I just want to make sure.
(Note: I believe a 6-wheel base would also make us accelerate slightly faster as there is less weight per contact point with the ground).