1. Rogers, McCabe, Coats conversations/run-ins with Trump in regards to Comey and the investigation don't qualify - for lack of a better term - to establish obstruction of justice with it being their word against Trump's. Comey's on the other hand does; since his subsequent firing - in which Trump personally admitted to being because of the investigation - can be used as evidence to argue the validity of his memos. Comey's testimony is to legally establish obstruction of justice. Means (The president can fire/replace the FBI director), Motive (To kill/stifle the Russia investigation), Opportunity (Trump unceremoniously fired Comey after realizing he wouldn't let the case go). Trump's guilt can be reasonably established.
2. Don't look for overwhelming bombshells in Comey's testimony - look for them later in the evening or the following day. Remember, we haven't seen the contents of all of the memos yet!
3. Sessions will be fired or will resign as a fall guy to soften the obstruction of justice argument to shield Trump as much as possible. It was Rosenstein's memo that was used as the original basis for the firing to shield Trump. Rosenstein clearly wasn't onboard with that plan and immediately denounced it and ran to the Senate Intel Committee to presumably implicate Sessions and/or Trump as being the reason for the memo's creation.
4. Trump's approval level is ~34%. When he's sub 30% the GOP will have no choice but to start actively distancing themselves for self preservation.
5. Once obstruction of justice is sufficiently established the leaks are going to get more damaging and we're going to see who's really loyal - self preservation.