to be able to face a commander sanji and zoro would need to be at gear 4th level, which they are not. It doesn't matter that we never saw them go all out, because what ever their all out is it has to be less than gear 4th. and from what we have seen you need the fire power and defense on caliber of gear 4th.
also there is way to many characters on luffy side who are clearly at a similar level or above zoro & sanji, they aren't getting more than one fight this arc.
before anyone cite the example of enies lobby, consider how we never had an arc like that for 13 years, the power-ups were sloppy and Oda never did it again. and zoro and sanji always fighting the 2nd and 3rd strongest is vastly over blown trope amplified by filler and movies, only happening in canon 2 or 3 times. EL & Alabasta are the only ones coming to me.
their gonna have to settle for tobi roppo this arc.
I think Marco and jozu are more appropriate, it ties to their connection to oden and it would be a great send of to the whitebeard pirates since they have taken too many Ls lately.