I'm trying to figure out how the First Amendment comes into play in the instant case or in the Garcetti ruling.
In the instant case it was not "free speech" but compelled testimony. Failure to testify would be obstruction of justice.
In the Garcetti case http://en.wikipedia.org/wiki/Garcetti_v._Ceballos there is at least some question as to whether or not Ceballos was exercising his right to free speech or exercising a magisterial duty when he wrote the memo questioning a warrant. But since Ceballos never raised that issue and SCOTUS never bothered to think of it, it does not matter - except to serve as a warning to others to cover all your bases.