My 8 year old has a July birthday. He started kindergarten when he was 6, and it was the best thing we could have done for him!
For us, it wasn't just about academics, although the fact that she's struggling would make me seriously think about repeating kindergarten. Our son was struggling socially. We didn't want him to always feel younger than his peers. The kids he went to preschool with all seemed more mature then our son, and we didn't want that to be the case. We wanted him to be in the same class with kids closer to his age. We just didn't want him to always be the baby.
Our younger son is going through the IEP process, and I have to say if there is any concern that she might need a little extra help, the sooner you start the process the better. School have resources, they have specialists in many areas, they have aids who could help your daughter in teh classroom. It's really important to get her that extra help if she needs it, and the sooner she has that, the easier school will be.
I strongly encourage you to go to your daughter's teacher and request an evaluation (the beginning of the IEP process). Personally, based soley on what you said here, I would probably have her repeat kindergarten so that she's in the same class as kids her own age. That extra year of maturity will really help her. But you have you have to feel right about your decision. Talk to your husband, talk to the teacher, maybe even talk to the principal, talk to your ped.
For our son, the decision was a no-brainer. For others it's not quite so obvious. You have time to think about this, so take the time and make sure you make the decision that feels right for you and your daughter.