How do I handle a high performer, with a really poor attitude?

Its seems like a straightforward case of giving feedback both positive and negative. You need to break down her attitude into behaviours, things that you can see/hear

It sounds to me like you may need to clarify what you mean by performance.  No one works in a vacuum, and the ability to work with and through other people, in my opinion, is as much a measurement of performance as detail oriented, good merchandising, etc.  It is harder to measure, perhaps, until you do as MI5Mark said and break down the attitude into behaviors.  If this person works directly with customers, then those people-oriented behaviors become crucial. 
That said, I think we all have dealt with someone like that, and it's far easier to give advice to someone else than it is to actually deal with it.  Often, people have no idea how they come across to others, and can be quite defensive when it is brought to their attention.  That's where the MT feedback model is so useful, because it's not controntational.  In the "here's what happens" part, try to tie the consequence to what the person would, or should, be trying to achieve.
An example:  an employee has a point of view or suggestion, but doesn't bring it up respectfully.  Instead, he verbally attacks other people's ideas. His otherwise good ideas are masked by his attitude. In giving feedback, I told him that when he does that, what happens is others become annoyed and don't examine his ideas seriously. 
Help your employee to see that a change in behavior can bring about a positive consequence.  If you haven't been giving feedback, then I wouldn't worry yet about documenting performance as per HR regulations.  Identify behaviors, clarify expectations, give feedback.  If that doesn't do the trick, then move to documentation.

 See the podcast, "How to manage an arrogant producer."  It gives great guidance.  It requires some very detailed behaviors on your part, including the use of the feedback model, which some managers find difficult to fully implement.  They also recommend a solid 6 months of working with them using this method, and that's only if you're doing it correctly from the start.  If you're not adept at feedback or some of the other behaviors they suggest, you probably want to get those down before starting this process.  If you're looking for a "Silver Bullet" to fix this problem, there isn't one.  But if you follow this process you'll feel good about the outcome, whatever the employee chooses to do.