Without going into much proof and needed detail, it seems to me there is no difference between Sefardi and Ashkenazi Jews in following the Rama, like the Ben Ish Chai implies. The same position appears in responsa Shevet Halevi volume 3, chapter 1. If Tosafos have a valid point on Gemara, why not apply this to halacha, as well? The Rama is largely a compilation of Tosafos and a summary of those who did the same.
Sure, there are various communal customs in prayer, and occasionally real dispute between Rama and Beis Yosef. That is not what I speak of. And even Ashkenazim don’t always follow the Ramah when he is disputed by, say, the Magen Avraham.
See also Bi’ur Halacha (on Mishna Berurah) chapter 219 s.v. Utrei.