I very recently had my audition with SCV and made it into the bari/euph line. Most of the applicants had previously marched in the open class or the non-finalist level of open class. It seems to me that being in an open class corps not only makes you look better on paper, but it will also improve your playing, marching, and understanding of what a drum corps is. You can get into any corps with talent and respect, but open corps will improve both.
I marched Velvet Knights last year, and my experience helped me out tremendously in how I performed at auditions, but remember, hard work (in practice and rehearsal) is really the only way to get to a top corps.