I don't think anyone would say they are likely to pose a major threat to Juve this season.
The debate is really more about what order Napoli, Inter, Milan, Roma, Lazio and Fiorentina will finish this season.
Signing Higuain has to push Napoli further up towards the top of that chasing pack.
I agree with what you are saying about team chemistry and the new coach/system.
No matter how good a team/squad Napoli assemble this summer, they are still going to be a largely unknown quantity and hard to make a prediction for.