Yeah, It does make a difference. 
I have always left the captioning on AT ALL times on the TV, even if I'm not watching it. My mother does the same thing too. Even if I'm not at her house, She always leaves the captioning on. She said she finds it easier to read the captioning because sometime (according to her) when someone talks too fast on the TV, she would not be able to hear all of these spoken words. So, Captioning helps.
That's a good question.
I have no idea if it will help with the foreign languages that are shown on subtitles but I believe it could help. It doesn't hurt to try.
You're right. Reading books helps with reading comprehension. With captioning - There are times that I've came across some vocabularies that I had no idea what it meant. So, Whenever I see a word that I don't know what it is for - I'd memorize it and then look it up on the dictionary to understand the meaning of that word.
