I wouldn't be too sure. She fought those heartless day in and day out, regardless of it being two years or eleven. She probably has mad skills by now.
Having her as an aloof ally in KH3 would be a good way to balance her being hella strong and not have her steal the spotlight. Sort of like what they do with Mickey.
Basically this, Aqua is definetly stronger from not only being a master but surviving two years in the RoD. If she appears sometimes in the game and helps out I'll be fine with that but having her as an ally throughout the whole game or even most of it for that matter is makes it unbalanced and the enemies would be afr too easy.
On a different note does anyone think that we will finally see variations to Riku's keyblade(if he has any)?? I'm really hoping so, his will probably have some creative designs with unique abilities.