I’ve been experimenting with trimming “I must obey my master!” while also incorporating Sir Ridley’s “Your mother once thought as you do” (albeit slowed-down). What prevented me from deleting “You don’t know the power of the Dark Side” was both Vader’s turn and making sure that Luke’s follow-up line made sense. The edit is a bit rough, but I’m sure Ady can do better working in the 5.1 mix.
The “I must obey my master!” line you deleted is very important. It tells the audience Vader’s relation to the Emperor, as it tells a lot about the Sith master-apprentice relationship.
Deleting it is a huge mistake.
Strong disagree on this one.