@Vialli, definitely get 7.4v Li-Po's [1300mAh or higher that will fit]
...and a good charger. (SkyRC S60)
Of my 16 AEGs, only one has a MOSFET.
All use 7.4v except the Lonex BAW which uses 11.1v because of the recoil system but it doesn't have a MOSFET.
The main downside of not using a MOSFET is arching on the trigger contacts will eventually damage the surfaces possibly to the point of not making good contact anymore but that would take a VERY long time.
Using 11.1v will obviously accelerate the process so I'll have to keep an eye on that but even so it will take a long time to have a noticeable effect.
Pulling the trigger slowly is worse for this situation.