Since your player has been using the 2018 Easton Ghost X -8 USA Baseball Bat: YBB18GX8, I would suggest going back with that if he liked the bat and had success. Changing to a -10 or -5 from a -8 would be better to do in the off-season and not right before some of the more important games of season.
The 2018 Easton Ghost X -8 USA is only made in a 2 5/8" barrel model. The 2 3/4" barrel model is USSSA certified, not USA certified. In terms of performance, the 2 3/4" model will perform better because USSSA bats outperform USA models, however some leagues require USA bats in 2018, such as Little League sanctioned leagues.
The biggest difference between the two bats is how they are constructed. The 2018 Easton Ghost X -8 USA Baseball Bat: YBB18GX8 features a two-piece, fully composite design while the 2018 Easton Beast X -8 USA Baseball Bat: YBB18BX8 features a one-piece, alloy design. It is hard to say that one bat performs greater than the other because every player is different. However, I will say that I have seen higher product reviews on the 2018 Easton Beast X -8 USA Baseball Bat: YBB18BX8.