I can't speak for all stutterers, but I would not feel comfortable speaking with someone who knows that finishing my sentences bothers me and chooses to interrupt me anyway. It's rude and inconsiderate even if you have little self control. Especially since you can control it, unlike our stutters. I can forgive ignorance, since lots of people don't encounter stutterers often, but I cannot forgive it being done knowingly. So if someone told me that pre-apology, I would simply not want to talk with them.