I think high average Jewish IQ may predate the Sephardi/Ashkenazi split.

Sephardim had Ashkenazi-level achievement in Spain. Italian Jews (who belong to a third group) dominated journalism and were wildly overrepresented in academia, the upper echelons of the military, and other high-status professions. When Sephardim came to Britain in the 18th century they produced figures like David Ricardo and Benjamin Disraeli. Maimonides and Moses de León (author of the Zohar) were Sephardim. Moshe Chaim Luzzatto (arguably the most important religious philosopher besides Maimonides) was Italian. Spinoza--the most important pre-20th century secular Jewish philosopher--was Sephardi.

There were historically unique opportunities to manifest genius in early 20th-century America and Western Europe. By that time, the Sephardic population had been decimated through intermarriage (they started mass intermarrying 100 years before Ashkenazim) and the bulk of the population was concentrated in the Ottoman Empire.

Would Einstein have come up with the theory of special relativity if he had been running around Istanbul wearing a fez? Hard to say.

Hopefully will be resolved with ancient DNA.