Yes, you can and definitely should submit before taking the August MCAT. You want your application and secondaries to be complete and just sitting in med school files waiting for your scores when they are released in mid-October so that your app can go immediately to review. You can send your transcripts to AMCAS a few days before you submit without problems, but you don't want to give AMCAS any greater than usual opportunity to lose your transcript (something they seemed to do with amazing frequency last year!), so don't send them too far in advance.