Sun Breathing is the original, most powerful breathing style, but we've only seen Yoriichi use it effectively. If a modern Hashira somehow mastered it perfectly, could they one-shot an Upper Rank like Kokushibo or Doma? Or are the Upper Ranks just too durable?
No way. Yoriichi was a different breed. Even with perfect Sun Breathing, a modern Hashira lacks his physical prowess and innate talent. Upper Ranks are tanks.
I think it could seriously injure them, maybe even decapitate an Upper 4 or 5. But Kokushibo and Doma are a different league, they'd regenerate from anything but specific conditions.
A full-power Sun Breathing could definitely do serious damage, maybe even a 'red blade' effect. But killing them instantly? Probably only if they hit a vital spot with supreme precision.
Yoriichi's Sun Breathing was so strong it practically made Muzan cry. If a Hashira could replicate even 80% of that, they'd absolutely wreck an Upper Rank. It's the ultimate counter.
Maybe a single hit wouldn't kill them, but it would certainly put them on the defensive and give the Hashira a fighting chance, unlike other breathing styles.