Ni Gorilla's arguable, but Asura was most definitely a significant power up. One attack in Asura turned Kaku's Rankyaku: Amane Dachi into mist, an attack that cut completely through the walls of the Tower of Justice and made everyone that witnessed it shit their pants.
Do we know if Zoro could have dealt with Amane Dachi at a higher diff using any of his other techniques, how about this very technique he used as his serious fight finisher in thriller bark against Ryuma?
Zoro didn't use asura against Oz or Kuma either when he was ultimately desperate, that leads me to believe his attacks are situational and Asura cannot be applied to all santoryu techniques, and that in fact santoryu is not always the best option anyway (as we see in this very chapter).
I am not shitting on Asura, I'm saying you are shitting on his base if you think it was that much of an improvement.
Luffy is using ryuo. Stop lying to yourselves.