Aside from position of weight, I would add that what you really want to reduce is the the rolling resistance of the car. One of the first areas I would target is the contact surface area of the wheels with the track. I would also reduce the amount of weight at the edges of the wheel. For instance, instead of using wide, fat wheels, I would use wheels that taper down into a v-shape.
This will minimize the energy being used to spin up the wheels, as well as reduce the energy being transferred through friction to the surface. I would also do whatever it takes to reduce friction in whatever bearing mechanism there is between the wheels/shaft/body of the racer. In addition, I would work to reduce as much weight in the wheels as possible. Again, the energy absorbed by the wheels needs to be minimized as much as possible.
So think along those lines...minimize rolling resistance, mimimize wheel weight, minimize wheel weight at the edge of the wheel. Maximize velocity at the bottom of the ramp.
I will add one little caveat, after looking at the track, there is an optimization problem here between energy lost due to friction, and the need to store energy to keep the vehicle moving. A spin up of the wheels will store energy, which may be needed if your friction is high enough that you need that energy in the flatter portion of the track. In any case, think in terms of transfer of energy from potenial to kinetic, what is stored on in the wheels, and what is lost due to friction, air resistance and rolling resistance.