Side-by-side comparison of Dourif (Legion) and Miller (Exorcist III)
If you're interested in seeing how Brad Dourif compares to Jason Miller, and then to himself in the re-shot scenes, I edited together a split-screen comparison to see how they fare side-by-side.
Check it out here: https://www.youtube.com/watch?v=Fexphlr9gTE
It's really interesting to see not only two different actors delivering the same dialogue in different ways, but then one of the same actors delivering the same dialogue in re-shot scenes. Dourif's performance is a little bit different in each version and it's hard to say what works better... I guess each works the way it should for the respective versions of the film.
Personally I like Jason Miller's inclusion and I have to admit I missed it when watching the director's cut.