If McGill doesn't offer your program naturally you wouldn't go there. However, in almost any other situation you'd be foolish to go anywhere else as a visiting to Quebec. It is hands down the best university in Quebec and this is coming from someone who didn't go there.
I don't think you'll have to send your high school transcripts to either OLSAS or LSAC. Those grades will likely be omitted from adjusted GPA calculations.
What if your program is Economics, Politcal Science, any fine arts major,or business? All of these areas McGill is surpassed by another Montreal school. Political science at McGill? Puh-lease. Fine arts? They dont even have a program. Business? Maybe if you want to teach after you graduate...
The only reason you SHOULD go to McGill is if you are studying hard sciences, philosophy, history, or linguistics. If not youre just asking to have your GPA deflated for no reason whatsoever.