Krias Shema is d’oraysa; therefore, you should say it before the MA’s zman (safek d’oraisa, l’chumra). So ruled Rav Schach, Rav A. Kotler, the Steipler, and Rav Shlomo Zalmen Auerbach. Rav Schach missed the zman of the MA twice in his life, once when he was 9 years old, and the second time when he was much older, and wept hysterically!
As far as S”E is concerned, the aforementioned Rav Auerbach zatzal was particular to daven S”E before the zman of the M”A, and it is praiseworthy to do so.
In both cases, if you rely on the Gra/ Baal HaTanya, you are not a sinner.