Generally, only about 46% of users who visit "ask a question" go on to post. For our new user pool the average was closer to 32%, which isn't too surprising. We all know that asking a question on Stack Overflow comes with a bit of fear and loathing (another topic for another day).
More interesting, there was a 3% reduction in questions asked by users who were exposed to the template. I think this is actually a really good thing. There's no way to know exactly why people who saw the template chose not the ask their question.
I want to point how that the search that powers "Questions that may already have your answer" is quite a bit stronger then the normal search bar. As an experienced user I commonly start writing a question just to get a match there. New users might be doing the same thing; with a little guidance, they have better titles or questions and it then shows a related question that solves their issue.