Impact from Zacks death? When AC came out we barely knew anything about Zack, we were barely emotionally attached to Zack in the main game, so it can't really effect the "impact" of his death.
I thought it heightened the impact of Aeriths death, it finalized that she was dead, and she couldn't be there with the rest of of the team.
I do kind of agree that AC did lessen the impact of her death. I think she had way too much screen time. They should have just gave us subtle to obvious hints that she was in fact present and then give her screen time at the very end in the church. The flower field, drops of water, and great gospel should have given us enough clues that she was present in the movie.
Though, the end of FF7 did leave up a lot for interpretation and AC did ruin much of that.