The trend I've been seeing is the opposite of what you've seen: many people are delaying the start of their children. We have a boy with an August birthday and we waited a year to start K. He would've just made the cutoff of Aug 31. He was bright at a young age in math and reading. We talked with lots of elementary teachers from around the state and country, and they all recommended this strategy. He's now in 6th grade and we've NEVER regretted it! He also has classmates who have June or July birthdays who waited to start, so he would've been the youngest kid by far, if we would've started him on time. (your daughter would be 1 1/2 years younger than these kids) We've always been very proactive about talking to teachers about him, and they've worked with us to keep him challenged. When I volunteer in class, it's extremely obvious who the "young ones" are, even when they're very bright like your daughter. Socializing is so important at a young age! (1st grade was our least favorite, but partly because we weren't as proactive on the first day -- learned our lesson!)
A couple other data points teachers gave me:
===When your child starts high school in 9th grade and is facing a lot of issues such a drugs, boys, etc., she will be 13, in a world where a lot of seniors are 19. An extra year of maturity can only help with making good decisions and dealing with the peer pressure of high school.
===Another year will also help in such events such as Spelling Bee, Geography Bee, and Valedictorian. These things are contested by grade, not by age. You don't get extra "points" because you're younger than the other kids in the grade. The better she does academically, the more likely she'll get a college scholarship.
You might want to consult with some of the ALP (honors) teachers in 4-6th grade, and/or Jr/Sr High School and get their opinions. If you decide to wait, sounds like you should find another pre-school who can accomodate a wider variety of abilities and keep her challenged -- there are lots of options. There are also activities outside of school to stimulate her intellectually, while she's getting quality socializing with her peers at school. Good luck with your decision. Take care.