Shulchan Aruch Orach Chaim 620 says that one should daven Musaf before the seventh hour of the day on Yom Kippur even if it means leaving out Piyutim and Selichos of Shachris. Mishna Berura says to even leave out Avinu Malkeinu if necessary. The Tur there implies one should even ensure to start by 6.5 hours into the day.
Some Chasidim do not start Musaf till later than that and on Simchas Torah even many non Chasidim do not Daven Musaf prior to the seventh hour. Is there any reason for not keeping this recommendation on the Shulcan Aruch? Is Simchas Torah in the Diaspora more lenient since it is Yom Tov Sheni?