My sister held my nephew back a year in K. They told her it was her choice (he has an Oct. birthday). The school said if she held him back, he would be a leader rather than a follower all thru school (as he would be the oldest in K rather than the youngest). Fast forward to high school where hs is doing quite well. He is a football and wrestling star (won the state championship). By holding him back, he has another year's growth on the other teens in his class. That wasn't the intent of holding him back but it was an added bonus. He is very well liked by the student body. His brother, two years older, graduated high school and has no ambition to go to college. He is working but nothing he will ever make a fortune at. He told his mom he wish he had been held back - he thinks it would have helped him mature and make better decisions.
). If I get tickets fine, if not, oh well. Hopefully my DH will come with me so he can drop me off and go park somewhere while I'm in line. 
) and he's not in the top grouping. He's high and does go to a different K class for reading group, but there are higher kids in his class and in kindergarten. From what you describe, you boys would be right in the middle in our K group, but they would also be on the young side since our cut-off changed from 12/31 to 10/1 and next year will be 9/1. Sounds like your boys are doing a good job, but you don't want them behind in a new school.
So, technically I was two years behind everyone else when I started college... it was an outstanding experience, so I think you just have to realize there are extremes everywhere...