I think he can become good in Juventus, and I wouldnt mind that honestly. The only thing with him is that he lacks brain as you said yourself, and is inconsistent, so he reminds a lot about Cuadrado.
He have moments of brilliance where he can look as the best player on Earth and others where he is the worst trash. Thats what you get with him. If Allegri/Juventus can make him more constant he is gonna be a great addition for you guys.
That said I definitely wont complain that we sold him and got James in instead. They are not the same kind of players though but to our offensive I belive we made a great deal in James
Good luck to Costa in Juve