I'd personally hate to lose even more grain than we already have from the Special Edition's Lowryization process. A little grain filtering is already baked into the SE because of that! If anything, I'd imagine a proper restoration from original negatives would have more grain than we're currently seeing. IIRC Harmy does add fake grain to his low-grain sources (GOUT, matte paintings, etc) so that they aren't quite as jarringly different from the surrounding scenes, but I don't find anything to be too grainy--in fact, I'm always pleased to see how much grain the film has retained. Grainy film is IMO an essential part of the 70's aesthetic; I'd probably find it distracting if there were noticeably less of it.

Also, low-res sample clips may have extra artifacts from heavy compression, which won't be in the final product.

EDIT: You'll also see more grain in brightened scenes (the ground in the redone sunset, for example)--but that's not because grain was added, it's because the Blu-ray hid a lot of grain by making scenes overly dark, and Harmy's just revealing it again through brightening.