Share your experience!
Tarball with kernel sources is good, but git will be better.
For example i downloaded sources for Xperia L 15.0.A.1.36 but how to guess last commit from which Sony branched out from main repository?
I suggest that you turn to our Developer World site in regards of this question.
http://developer.sonymobile.com/about/how-do-i-get-in-contact/