Hey Stacy -
This is a tough one for sure. I have two boys - one was born in May and the other in September. I remember having the same concerns with my September son for many of the same reasons. However, he has done a fabulous job acclimating to school, its rules, and the other kids. (He is now in 1st grade). So somehow the structure of school actually helped him.
However, my May son has struggled academically since the beginning (he is now in 6th grade). He is much more sensitive and notices when people are upset, etc. I have been struggling with teachers all these years about holding him back, which they never seem to want to do - and therefore he is still struggling. He is NO WAY ready emotionally (or academically) for Junior High next year. I wish I had not started him in kindergarten until he was a year older.
On a side note, my birthday is in December and I missed the cut off by 3 days, so I started school when I was nearly 6. This was never a problem for me - in fact, in HS I thought it was awesome that I was one of the oldest kids there.
I know I really didn't answer your question, but I hope my post helps you a little.