Im going to give you a example of what happens if you use a 6v total batteries on a CF5, since my graflex is currently running on 3v batteries:
As far as i can tell, the only loss is on the sound, the sound will be as loud as a hasbro fx saber, which compared to my other CF sabers is very low, i cant say if the low voltage also affects the brightness since my P4 looks extremely bright, so my guess is that the low voltage only affects the sound.
There are some 3.7 batteries but you will have to make a choice between v and mAh, and as ive been informed, mAh gives you more run time on your saber, so as far as i can tell is power (v) versus run time (mAh)..
So with 3.7 lit-ion batteries, you will have enough power for led and sound, but 3.7 batteries give you less mAh so the run time is less. On 3v batteries you have around 1400 mAh so your saber will run for about an hour before you need to recharge, but you wont have enough power to have a loud speaker, maybe even a dim led.
Conclusion: you cant have it all.