You know, watching this clip here:
http://www.youtube.com/watch?v=zCEQLvOW4cM&feature=related
It is not so much the "flashback" stuff I would include or comment on, but the end fight between Luke and Vader (4:33 to 5:08). Whether Luke's eyes go "Sith" (at least until the Emperor confronts him and he regains his composure) or not, SOMETHING has to be done to explain Luke being able to overpower Vader. Hell, it also looks like David Prowse just LETS Luke womp on him!
Ady: maybe you could add some wind audio FX, modify the scene(s) from EMPIRE where Vader starts pulling stuff off the walls and have Luke pummelling Vader with pieces of equipment and metal as he's attacking him with his green saber (so Vader is fending off Luke's saber attacks while getting hit with chunks of debris). At least that way you would visually see how Luke overpowers Vader.
Also, is there any way to snazz up the Emperor's room? Does anyone else feel like it looks like a cheap set? And when you get around to JEDI, consider taking out some of those really bad and boring overhead shots (2:18-:19). Those shots look like some 2nd unit director work from a Sci-Fi movie of the week.
Also also, could you edit the dialogue a bit at 3:54 - (cut out: ..., especially for ... SISTER... so, you have a twin sister... your feelings have now betrayed her too. Obi Wan was wise to hide her from me. Now his failure is complete). Still keep: "If you won't turn, then perhaps SHE will." The reason I ask this is that this section of dialogue is pretty damn hokey, even for STAR WARS standards. Having Vader say "Your thoughts betray you. Your feelings for your friends are strong. (pause) If you will not turn, then perhaps SHE will." would be more than adequate and I think more fitting for Vader's character (not as goofy and more to the point)
The idea is that you still have the threat on Leia, but it's not so heavy handed and mustache twirling. Hell, maybe you could just delete all references to Leia being his sister (as Luke said in Robot Chicken: NOOOO!! That's highly... improbable!). When Han asks at the end, she just kisses him without the "bash over the head - I'm Luke's sister" dialogue.