Fastsky hit the nail right on the head. The results of higher voltage is going to be your servos moving faster. The servos will also draw more current which means you will use up the battery packs faster meaning less flying time. That's the trade off for getting more speed and power out of your servos.

I don't know about the other makes of radios, but Airtronics does not recommend using 6v (5 cell) batteries with their receivers. I once considered doing this and called and spoke with Airtronics about doing this and they recommended against doing this, also doing it would void the warranty too.

Ken