[BrianG]

Yes and no.

Think of it like this: you need X amount of power to have the vehicle perform like you want. Let's say you need 250w average (fairly typical). At 4s, you need 16.89A average. With 5Ah pack, that gives you almost 18 minutes of runtime. At 6s, you need 11.26A for the same power, and that gives you 26.64 minutes or runtime.

Now, this assumes you regear to achieve the same speed in both cases, and that you aren't exceeding the max motor rpm...