I´ll comment a little here from personal experience:
I took the MCAT August 20th of last year, applied broadly, had 5 interviews and so far only heard back from one so far but it was an acceptance. So is it a death sentence if you take your MCAT late? No it´s not, however I don´t think you want to do that if you don´t have to...
First off in my experience a late MCAT is fine for schools that have lower numbers of applicants....2-5000 applicants or something like that, however by doing that late MCAT, your application will be seen a lot later, and with schools that have higher numbers of applicants, that may mean it doesn´t get seen at all, or at least with the sort of attention you want....
Second, taking the MCAT in August means you´ve only got one shot to get it right, because you are sending in your application to AMCAS before you even know what you got on the MCAT. If you do poorly, you just wasted your money, and possibly a year. In my case, I was finishing my pre-reqs (I´m a non-trad) over the summer and so I wanted to finish my organic chemistry classes before I took the MCAT.
So to answer your question, no, it might not kill you to take the MCAT in August, but if there´s any way for you to avoid doing that, and still doing well on it, take it early and get that application in as soon as possible.
Good luck and study hard!