asura is portrayed as a power up to be as grear2/3 and diable jump, why would it be his greatest attack when luffy and sanji upgrades were just improved a little, while they developed better attacks?
Asura is just a crutch used to pretend like we haven’t seen zoro's best post time-skip. we have, that's why he got emma.
Against the pacifista the M3 used their strongest attacks Luffy, G3, Zoro, Asura, Sanji, DJ
Then in Fishman Island we got a similar scene vs the Kraken with Luffy using G3, Sanji using DJ, but Zoro used his Ougi or secret techniques.
We then go on to learn later that arc that Sanji actually gained a better attack than DJ called Hells memories
Luffy then goes on to reveal G4 in DR
So since his Ougi was portrayed as his DJ, G3 Equivalent it is only natural to assume Asura is his G4,HM equivalent.