The C rating only determines the amps the Lipo pack is capable of delivering. There's 2 ratings on most packs, constant/sustained and peak/burst which is meant to be 5 seconds or less. You simply multiply the C rating with the mah rating, example a 20c continuous 5000mah pack is supposed to be able to deliver 100amps continuously.
The motor, vehicle weight, tire size, gearing, etc will determine how many amps your system will draw. The system will try to draw this from the battery regardless of the batteries capabilities. So if your system will pull 100amps continuous, a 20c 5000mah pack is sufficient. A 5000mah 15C pack will not be able to supply enough amps and may end up lowering motor temps, but the lipos temp will be higher. A 5000mah 30C pack will just not be pushed as hard and have some reserve. I bring this up because you do not gear according to the C rating of your Lipos. You gear according to motor and esc temps. The only time you adjust gearing according to a Lipo pack is if your Lipo is getting too hot. Then you can lower the gearing saving you from buying another pack. Or in the case of your racer friend, you can up the gearing because the pack can put out more amps. But that's assuming the temps on the motor and esc aren't too high or he just doesn't mind pushing them higher.
Btw, I hate to sound like a know it all or contradict anyone, just making sure anyone reading this understands the whole picture and doesn't get the wrong idea.