I haven't ever seen any stats on acceptance rates, although they may exist out there somewhere.
I don't know of any school that makes a distinction between scores from April vs August, but what can make a difference is when you apply. If you apply to a school with rolling admissions (where they accept some students throughout the interview "season" rather than waiting until they are done interviewing), then your chances are theoretically better if you apply and interview earlier because there are more open slots.
:twocents: My recommendation is that if you are going to take the August MCAT you should get your centralized application and transcripts in either before or immediately after taking them MCAT. That way all you'll need to complete your application is the MCAT scores. Once they come in you'll be ahead of the game for the August MCAT-takers who waited for their scores. You also won't really be far behind the April testers.