You can submit AMCAS without your MCAT score, you simply need to check off the box saying that you are taking it in the future. You're application will then be put on hold until MCAT score received. I was originally going to do the same thing, since I'm not happy with my April score and retaking 8/6. However, after talking with some people here, with my low ugrad gpa I didn't feel comfortable not being 'under review' until mid-september (when they get my new mcat score and when secondaries are sent in), especially since my state schools' deadline for secondaries is October it would've been very late in their cycle. With that in mind I moved my test to September and decided to apply next cycle.
Just more food for thought. If you have good numbers and experiences though, completing AMCAS ahead of time, and taking in August will not put you at a significant disadvantage.