We all read Comey's testimony ahead of time, so it's not a bombshell. The really good stuff Comey's holding back, that's what the GOP is worried about. The stuff he's telling behind closed doors - in camera - is what will hang Trump and his entire group.
If we ever get to hear or read that, it will be several years in the future. The Watergate hearings were given in public (on national TV) but there was also private testimony that we never saw.
So the GOP is worrying about how they're going to spin this. And spin they will.