I find myself agreeing with your conclusion while disagreeing with how you got to it.
Kiffin is more established than you think. He has been a head coach for 11 seasons over a 14 year time frame, spanning both the NFL and college ball. That's only a handful of years less than Fulmer was a head coach.
I would not call him a journeyman. He hasn't been that since his stint at USC. Between Oakland, the Vols and the Trojans, he had his "growing up" years. At this point, he is what we see. He will continue to grow a bit as a coach and person, we all do if we're trying, but the grand arc of his developmental years is complete.
Conclusion: he is not, and never will be, a championship coach in the Power 5. Second-tier, sure. Never close to Coach Fulmer's accomplishments.