In response to this http://dinonline.org/2010/06/02/time-for-krias-shema/ – I would like to note that the following poskim insisted on following the earlier zman of the MA: the Gadol Hador Rav Schach (who wept hysterically when, on two occasions in his life, including one that occurred when he was a child, he accidentally missed the zman of the MA), Rabbi Aharon Kotler (who rebuked bachurim in his yeshiva for relying on the later zman), the Steipler Gaon (who said safek d’oraisa l’chumra), Rabbi S.Z. Auerbach.
It is interesting that in this particular case the Steipler Gaon argued with the opinion of his rebbe, the Chazon Ish.
The general trend today is that the Yeshivos follow the Magen Avrohom’s zman. I am sure that the Brisker Rov and other Gedolim were mukpid on the MA zman even though they held that the main zman is that of the Gaon. Perhaps Rav Shach was only muchmir on himself?