Probably very substantial. I mean, you guys wil be getting your MCAT scores in, what, mid october? Compound this with the inherent slowness of AMCAS. The most prudent of us april test takers will have finished all their supplementals by then, and some might have interviewed by then. Honestly, my inclination would be to say, while not total, august mcats will hinder your application considerably.
Basically, have as much done as you can before the scores get back, definitely pay the extra 15 bucks to view them online early, and be ready to pay for express mail. It seems like every day counts in applying. good luck