As a general rule, you should gear the car so that speed tops out just as you enter the turn at the end of the longest straight on the track and tune for infield performance from there. You should be able to judge that pretty quickly and make the necessary adjustment. Obviously, it's best to start with a higher gear ratio (smaller pinion) and increase one tooth at a time. The difference between single and dual shunt brushes are that you have two braid wires on the dual shunt brushes vs one braid wire on the single shunt brushes. Each braid wire is capable of carrying 10 amps of current, so, if your hoods are really dirty to the point where they have intermittent contact with the brushes, you are able to pull 20A through the dual shunt brushes vs 10A with a single shunt brush.