Well, using a 4.8V 5000mAh receiver battery pack in your plane won't make your servos put out higher torque. This battery pack only gives you longer flight time.
Totally different situation. The servo will only draw so many amps with a load applied. Supplying more amps than it can possibly draw does nothing except give you longer flight time as you've stated. A starter motor is another story. If it needs more amps under a given load than the battery can supply, it won't be able to do the job. Based on what you are thinking, you should be able to start your car with the same 12 volt battery you use to start your model engines. That ain't gonna happen.