I'm not so sure about the theory that composite bats are neutralized by the RIF (reduction in force) safety balls even though I have heard this mentioned before.
Not scientific, but when DD was 8YO I bought her an entry level composite bat, Synge, and she could hit the ball about 15 to 20 feet, noticeably farther with it. She has previously been swinging a cheap, Worth aluminum bat. All things being equal, a composite bat will improve performance no matter what type of softball is being used.