I like Isner personally as he seems like a decent kid, but no way he is a dominate player over a slam tournament. He might make the quarters and he might spring a big upset, but he is not going to beat 4 or 5 quality opponents in a row in best of 5 set matches. Great serve, good ATP level FH, slight below average BH, movement below average which impacts his FH and BH in rallies, slightly below average return, and slightly below average volleys. Australian courts are not the best for his game either as they traditionally are slower than Wimby and the USO.
You cite wins over Federer and Djokovic but those are like career days for John. I have also seen him lose to others ranked outside the top 10 make he look very vulnerable. On some days, he looks like he cannot get 3 or 4 shots in a row in the court.
Round of 16 is good tourney for John and quarters would be exceeding expectations.