Again, this assumes each dungeon has a large amount of unique art assets. The way it will probably work (and how it worked in morrowind, oblivion, and FO3) is that they have a big list of chunks that they can fit together to make a dungeon. One piece of a dungeon could be used in many, many other dungeons, but by arranging them differently you can get dungeons that feel unique. The actual information that specifies how the dungeon pieces are laid out in a given cell takes up very, very little disk space. So I think this would only really work with dungeons that truly are completely unique (which would probably be the ones associated with major quests), or by removing whole categories of dungeons that use the same assets (dwemer ruins, caves, nord forts, etc)
This. The reason for the repetitive nature of OB dungeons was the lack of mixing dungeon sets. For example Caves pieces were always with caves pieces rarley mixed with forts or Aylied ruins. Simply by mixing assents more you can create dungeon that feel a lot different from each other. I was playing Nehrim and a number of dungeons had only original assents yet felt unique compared to OB's because they mix assents, caves, forts, even oblivion realm stuff together. I think this will be the approach SR uses. Ad for dungeon types, OB had 5: caves,mines,forts,Aylied ruins and ob relm. MW had caves,strongholds,dadric ruins,Dwemer ruins and perhaps another one I can't think of. SR might have 1 or 2 more, but its going to be the mixing that matters most.
As to the op's concerns. OB still had about 2GB of space left, so I am skeptical of there voice cutting claims. Of course more VA means more space even for the same amount of dialog. Not with better compresion tecs and the extra GB of space there should be no issues. Beth uses very little CGI in there games, which take up massive amounts of space. Though I do feel that the 360 is being hurt by its lack of disk space.