Arts Supplement at top schools?

I’m planning to apply to a few selective schools. I’m wondering if I should submit an arts supplement; can I get some feedback on whether it will help my chances? I’m NOT going into music.

I’m a clarinet player (and a piano player, but that’s probably too common for me to stand out). I’ve been principal clarinetist in my school band since freshman year, and have participated in All-State bands as well as youth orchestras. I was a finalist in a concerto competition. I’ve been invited to perform in a lot of places and I have a few awards. In my state, I would be considered a very advanced player (most people say I’m about college-level, a few say graduate school-level).

-is clarinet a relatively common instrument?
-do I have to be Julliard-level for it to help my chances?
-can I send it to some schools but not others, or will Common App send it to all schools I apply to?
-will it harm my chances if I’m not outstanding enough?
-am I correct that I should not submit one for piano? I am equally good, if not better, at piano than I am at clarinet. However, I realize that lots of piano players are probably better than me.

I should probably add that my school of interest is MIT. Also, most my EC’s over the past few years have been music-related; I am very passionate about and invested in music. If there is a non-major music-related group in whatever college I go to, I definitely plan to join it.