I did a pretty good test on a couple of rifles a couple of years ago. And I didn't see any significant increase in velocity. I used it for breaking a couple of used rifles that were bad foulers and the break-in improved that a little but I wouldn't necessarily credit gun juice with it.
If you try it, be careful not to leave any wet juice in the bore as the instructions say you should. That is dangerous, unless you are using a greatly reduced load. I know this from experience.
In the long run, not worth the time, effort and $$ IME.
I'm going to give some custom barrels a melonite nitride treatment which i have read a lot of good things about. but that needs to be done on a new or almost new barrel.
Microlon Gun Juice Report