My kid was born in September and I remember my wife doing a little research around that "for fun" (yeah, that's how she has fun) a while ago, how kids far and/or close to the cut off do.
She found a few studies mentioning that older kids (like born between September and February or something) were doing way better in terms of the academics as well. Which in a way makes sense, but maybe indicates that either children start school too young or that programs are too agressive.