In high school I did Policy debate, but didn't really like it so I switched to IE. I did extemporaneous and impromptu speaking all four years of high school. I was state champion in both impromptu and extemporaneous speaking my senior year.
It has helped me tremendously in several aspects. The most obvious being that I never get nervous in front of large crowds, and can rock an interview without breaking a sweat (I know this will help me greatly when I apply next year to med schools.) It was brought up many times when I was applying to college and got several scholarships for doing speech. I'm also the top seller at my work place because I'm able to really connect with people, and can speak to them well.
I'm currently doing extemp and impromptu in college as well (NFL). Sharp communication skills are vital to being a succesful physician, and being able to articulate your thoughts and be a good public speaker are highly desired traits (confirmed by an adcom I spoke with a couple of months ago).
I would certainly mention it.