higher mah gives you 2 things... more amperage on discharge and more battery life. There's a discharge rating on batteries thats multiplied by the mah rating to give you the final amperage of what the battery can provide. But essentially there's some witchcraft that happens inside the cells, but a 400mah increase on a nimh is not going to hurt. Expect slightly faster trigger response/motor response and a bit more life out of the battery... depending on the health/quality of the cells.
Now say you put a 9.6v 4000mah on there made with those fatboy C cells... then you're talking a huge jump in rate of fire and trigger response because of the amps it can dump when you pull the trigger.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.