Emm.... talking about bounty posters... Don't you think usually the flashy ones get the bounty marked on their heads? Especially the captains... and first mates...
Just because Sanji isn't in Supernova list doesn't mean it does him justice to leave him out of the league. Zoro's first bounty is 60 mil (due to his former status and incident in Alabasta), while Sanji's first bounty is 77 mil (solely due to Enies Lobby incident). It's a given that Zoro's bounty goes up eventually, especially after EL (for 120 mil beli), and other big case since then have been covered up by WG so chances for their bounty to get higher have gone down the drain till they arrived at Sabaody. Sanji has the potential to be ranked among the Supernovas, again his low profile from the start isn't contributed much, and WG have no idea of his current strength & threat level.
Posing threat to WG doesn't necessary means the level of physical strength of the said pirate/individual which bring risk to WG and citizens, rather, it refers to the consequence/origin of the person's behaviour/deed/knowledge/lineage. And that's what bounty poster is all about. It's a tad subjective to use it as an article to the argument points for me.
Regarding to Sanji vs Capone (if that's how the event really turns out), no matter who wins in the end, the fight surely ends high diff. Unless Big Mom's crew giving a hand, for Sanji & co. to stall the time that long says a lot to their overall strength. Don't forget, before that Sanji has faced Viola's (and her subordinates') attacks with a slight handicap on his chivalry, and lastly fighting Doffy halfway. Nami's safety shall be one of his burdens during the battle, despite knowing Nami & co. can possibly take care of themselves.
Given that there are two trump cards with 'em; Caesar and Momo, Sanji may use a sort of strategy to escape from them, after putting up a hell of fight. He won't ask for permission from Luffy confidently to counter fire at Big Mom out of his whim.
Also, somebody tells me again why Zoro has something to do with Sanji's winning probability over Capone? Put Zoro in Sanji's place then, let him fight Pica instead, crushing that soprano stone guy to bits. Zoro can rush ahead Sunny and fight Doffy halfway... oh but without sky walk. But still... Zoro would manage, he can do long distance slash.