I don't know if I would delay sending in your application just because you are waiting for AOA acceptance. You can send it in to CAS as a supplemental document later (they have stickers that they'll send you for that). I was in the same boat. I sent my AOA letter into CAS asap when I got it (early October) and I took a copy to each of my interviews to make sure they had it. When exactly will you get senior AOA, early september is a lot different than late september? If you knew you would get it the first week of September it might be worth it, but not any later, in my humble opinion.