stay with me on this;
what if he put TWO, yes two CO2 cartridges, then he'd have like 10 seconds of extra time.
Or even better still, he could have the CO2 cartridges pointed towards a micro-wind-turbine generator and that would charge up the batteries. so when the car is running low, he can activate the CO2 and get more runtime.
I also hear that using the right tires will help. There are a lot of guys that use smaller wheels, that way there is less rotational mass, and since they are smaller they will spin faster, but require less battery to do the same work.