It sounds like a 504 is necessary. If everyone who works with her thinks that she needs one, then just do it. As a friend on mine said, if your kid needed glasses you wouldn't hesitate to buy them. It should be the same with 504 accommodations. Legally, the school can't discriminate against her just because she has a 504.
FWIW, I just attended a 504/IEP process review for 2e students through our school district. They said that since the they eliminated the discrepancy model (achievement being much lower than potential based on IQ and achievement testing) many 2e students do not qualify for IEPs. The key question to qualify for an IEP is whether the student can receive reasonable educational benefit from general education classes alone. If your child still gets As and Bs at a gifted school it is highly unlikely that she would meet this criteria.
