At the PUBLIC FISA hearing today, no one who testified wanted to go on a public record, that could be used by special investigator Mueller, as evidence of obstruction of justice, one of many predicates for impeachment. I'm pretty sure those actually following this know the "meat" testimony will occur in the SCIF, out of the eyes and ears of We the People.
Keep in mind, the reasons for those who would not testify in public had nothing to do with saving the lives of intelligence assets under the guise of National Security. Or exposing the means and methods of intelligence gathering. On the contrary, it has everything to do with how quickly this administration is fitted for orange or striped jump suits, or given the opportunity to resign. I can wait a few more hours I guess.
That being said, I believe the same thing will happen during Comey's public testimony. He is a civilian, but I believe he will provide nothing in public testimony we haven't already heard or already knew. It will be the testimony given in the SCIF. That is where the rubber will meet the road.
What will be key is that special council Meuller is present at both the FISA and Comey classified testimony hearings. If this investigation is to have integrity and credibility, it is imperative that he has the opportunity to hear the testimony and ask specific questions of those testifying, rather than rely on hearsay or 3rd party "evaluations" of what was actually said.