chances are what you area seeing are "Steering Speed" adjustment which is fairly standard on most computer radios. This feature is used to "slow" the response of a servo. Many drivers tend to buy the fastest servo they can get but this often can be a burden to where having too fast of a servo might require "over correction" in a turn, then they may need to slow down the speed to make the car easier to handle. There are some ESC's which come with a "selectable BEC" which can toggle the voltage from 6V to 7.4V and this can increase the speed of the servo, but this is typically controlled from the ESC programmer, not the radio, however there are some brands of radios (i.e. Graupner) which offer an integrated ESC which can be programmed from the radio where both the ESC and radio must be a matched pair for this feature to work.
It is possible to buy an external U-BEC to increase the voltage, but folks with HV servos will often bypass the red wire from the servo, and hook it up directly to the input lead from the 2S battery to get 8.4V voltage directly from the battery. Only problem with this technique is that the speed of the servo will slow down as the battery is drained and you get closer to LVC.
I prefer to keep my BEC at 6V and not have to worry about the servo speed fading during a run.
