I would suggest using the gun before you "upgrade" anything. If you enjoy tinkering, by all means, dive in: Starting with a whole wack of reading.
The individual cells of the battery are 1.2v each, in series 7 of the cells makes 8.4v. Each cell has 1100mah discharge rate.
If you gun shoots fine with 8.4v mini, you should just buy another 8.4v mini, as they tend to drain to the point of inefficiency after 700 rounds or so is my experience with them.
Good luck, and enjoy the sport.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.