It seems a little strange. Sanji's most powerful attack is "Hell memories" right? He already used his fucking most powerful attack in the fishman island, whereas Luffy only did use his trump card against Doflamingo and Zoro didn't even use it yet. It seems Zoro and Luffy are a big deal ahead of Sanji, to which I think this latter will get a power up (be it taken from the ass or no) to have him at least in an equal footing compared to Zoro. This saga seems ideal consedering it's focused on Sanji and seeing as all of his brothers are stronger than him and he is deemed as a failure. I think he will receive something at the end of the saga that will prove to his father that rather than being a failure he is the "most unique one".
Hell's memories is an attack used when tapping into the bad memories he had when a kid. He might be able to create an even more powerful version of this attack when tapping into his experiences as a kid, which seems worse than what he experienced in the "Okama training".