Luffy still has a long to go physically as well, actually. He knows he's totally outclassed by Garp and Garp's only a Vice-Admiral. Also, he's not quite packing as much power as the top tiers of OP yet. Admirals casually bust city blocks, if not greater - Luffy needed Gear 3 to break past a reinforced steel door. Admittedly that was a couple of arcs ago, but his improvement hasn't been that dramatic, and the difference is great.
Haki + arci mprovements would probably send him from, say Upper Middle Tier (seems a bit low, but consider that he beat Croc and Moria with severe plot), to Lower High Tier or even High Tier, but I'm pretty sure he's still nowhere near top tier. He's still got plenty of time to grow. If the SHs are anywhere near Admiral level the manga is going to be ending very, very, soon, which I doubt.
Sengoku is definitely stronger than Garp, he's the freakin` Fleet Admiral!