DO. NOT. TAKE. AUGUST. MCAT. DO NOT. DON'T. I took the MCAT in September. It DRASTICALLY impaired the strength of my application.
1. I was rejected pre-interview by my alma mater/state school/top choice, and I can say with a pretty high level of confidence that if I had submitted my application 2 or 3 months earlier, I would have interviewed and stood a relatively good chance of acceptance. My scores were above avg there, and the rest of my app was very solid.
2. I did not have the opportunity to apply to many schools that I would have liked to apply to, b/c by the time AMCAS finally verified my app, the deadlines had either come and gone or were with that day.
3. 2 of the schools that I did interview at, I interviewed for WL spots b/c of how late my app was.
I am happy with how I did in the application cycle. I have been accepted to two schools that I really like, and another I liked much less, and I'm on the WL at another school that I like with a relatively good chance of being accepted, I think. I interviewed at 5 of 11 schools I applied to, and was/will be accepted to either 3 or 4 of those. I do not blame my rejections on my late application entirely, but I know that at at least 2 schools, it was a major issue. (2 of my state schools.)
Some people may be unaffected by late applications due to stellar stats or other factors. MOST are affected. Don't apply late. My advice would be take the MCAT by early July at latest.