I think it really depends on the personality of the child vs. the type of kindergarten situation you are looking at.
My DS was 5 years 10 months when he started kindergarten. And is showing to be HG+. But I still think the full day kindergarten he went to would have been really horrible a year earlier. I think he learned more through self exploration and play based preschool that year than he would have in this highly structured-more like 1st grade environment that had really very high behavoiral expectations. My son is actually NOT a behavoiral problem at school, but is an active kid and was a particularly active preschooler.
On the other hand my DD who is 3, loves the structure part of preschool and is very cooperative with teachers even now. She might be a great candidate for early K, but has a summer birthday, so that won't happen.
