How much will a strong arts supplement impact your application chances?

I'm applying to Harvard-Berklee's dual degree program as a jazz musicain. Obviously a prerequisite is that you get into both schools separately.

I definitely have a conservatory-level ability (I've been invited to study at Peabody and Michigan State which are both very strong programs) and I likely would be accepted into any conservatory for jazz except for Julliard, just because julliard is so competitive. I would be shocked if I didn't get into berklee. (I hope I don't sound like a dickhead)

My bigger concern is getting into into harvard. I have strong activities with leadership, national awards, and a good ACT. My GPA/Rank is good although probably my weakest point. My rank is about 15% in a class of 600 students, mostly because my school is hyper-competitive. There's over 150 students with a GPA above 4.5, so to make the top 10% your GPA must be 4.7+ My UW is 3.88 and my W is 4.56, so not bad. Not great.

Since my application is still fairly strong and almost every spot, I'm hoping my music supplement will help sway my chances. How much influence does that hold? How does that whole process work? I will be meeting with Harvard's music people when I go to audition at Berklee.