If it's anything like my Avalon, it wont be the spring that'll be an issue when running a higher volt battery.
It will put more strain on the gears though so they will wear out quicker.
The only thing that'll improve when running a 11.1 over a 7.4 is trigger response and rate of fire, I believe, so it really just depends if you can justify the need for that being greater than the prolonged life of gears etc.!