I agree with you in general. Messi and Pep combo is the key, not so much Man City, though the quality and depth it has is a clear advantage for Pep. Pep knows how to use Messi and he will adjust both Messi and tactics to produce. It will be instant as you said, cause they both know how to do it, plus Man City is already like Barca 2, with more substance.
That's right, you are both right! I also agree, it is the combination of these two professionals that creates a powerful synergy: Pep and Leo Messi.
It often happens that a coach has his own reference players with whom he works very well: it is absolutely normal, indeed.