Brachos 13a - ברוך שם כבוד מלכותו

The magen avraham [61] writes that if one doesn't say ברוך שם כבוד מלכותו after shma, he need not go back and say it. The Levush rules that he must go back [to the beginning of shma as is implicit from the chaye adam in klal 21 or at least to baruch shem]. The Biur Halacha there proves that baruch shem is not critical from the mishna that says היה קורא בתורה והגיע זמן המקרא אם כוון לבו יצא which implies that all he read was what is actually in the chumash [which excludes baruch shem].

However one can argue that baruch shem IS critical but it just need not be said in its proper place and may be said after completing shma [and the din that shma may not be read out of order is referring only to what is actually written in the psukim and not baruch shem]. According to this, there is no diyuk from the mishna that baruch shem isn't absolutely necessary.

[גבורת יצחק עמוד קסח] 

This is an interesting topic worth revisiting. Now I must go daven.....:-)

Love and blessings!