Best place for any moveable weight is going to be low and between the wheels, as close to the middle of the car as possible. The factory spot is pretty good... It sortof balances the drivers weight being on the passenger side. Anything closer to the ends of the car increases the amount of energy needed to turn the car, and stop turning too.
I learned that much flying aerobatic Radio controlled airplanes for 10 years, and racing R/C cars competitively for 3 years.
One other suggestion I just thought of, but it's probably too late to change now is to have the fuel tank where the passenger seat was. That would eliminate changes in handling as the tank empties. I've seen this in other race cars and wondered why, but it suddenly made sense.