This boundman technique seems like it can be pretty effective if it lasts longer. It is possible he may also have another gear up his sleeve. The caveat seems to be that Luffy's current Haki can't sustain these advanced techniques.

Based on what Rayleigh mentioned about Conqueror's haki, I would guess that the way to increase ones Haki reserves would be by gaining more experience and not through training. There may also be some event down the road that may trigger Luffy's devil fruit awakening. Seems like Luffy has access to the tools needed to become alot stronger. He just needs more experience in the New World now and I think he will get to a level eventually to be able to fight with Kaidou. This fight itself is a good one to put under 'experience'. I am pretty sure Luffy will have better handling of Gear 4 after this arc.