It depends on the motor being used. For example, if you run a 2100kv motor on 4s, that would be within acceptable limits. However, if you run the same motor on 6s, obviously, there is much more strain on the ESC.

But lets say that you want to run around mid 30's in MPH, the setups are as follows:

2100kv motor with 4s(14.8v) battery = 31080rpm

or

1400kv motor with 6s(22.2v) battery = 31080rpm

So with the car and the gearing being equal, top speed should be the same. In the above examples the 6s system will pull roughly half the amps of the 4s system, which is easier on the ESC. It also allows you to run a lighter, low Mah battery and still achieve roughly the same run times. The two different setups will also drive differently, because of the different torque and power curves of the motors. These are just examples of what's possible, so don't flame me for missing something.