I think he has observation haki mastered with future seeing. Armamnet haki mastered i would guess is what vergo did. Imagine someone like kaido completely clad in armament haki and twice his already size. All buff. But luffy could get there. As far as kings haki being mastered... No idea if you can or not until we see more of it besides knocking out fodder. And making animals back off, Other than that id say he stays at gear 4.. Just being able to use it way longer, fully clad in armament haki , buffed up twice his already size, plus future sight seeing, and making his surrounding rubber so he can king Kong gun someone into the ground make them bounce back and then rhino schneider their ass lol