In truth, the only thing that I felt was missing from ROTS was a small scene (30 seconds) with Anikin in the temple killing a few adult Jedi. I would have love to see him wiping them out with no problem, showing us how strong the dark side has made him.
I think that the hologram didn't show enough.
You're almost right. It's not about what we actually see. It's about what we can imagine. In the hologram scene I think we needed a few beats of watching Obi-wan's reaction to the slaughter. That would have been very powerful on many levels.