My oldest son has severe ADHD, but we didn't get a GAC card for him. He was able to handle the line-waiting with no problem at all. We had one meltdown with him, and that was because he hadn't been eating much (believe me we had been trying to get him to eat), so we just got fastpasses, headed to the nearest counterservice and made him eat a jumbo hot dog.
After that, he mellowed out and didn't have any problems since. I'm still surprised that we only had ONE meltdown.
Perhaps your best bet is to sit down with your ADHD child and spell out what she can/cannot expect at Disney. To try to be flexible and spontaneous. I know that last-minute changes can wreak havoc on an ADHD child. At least mine, anyway. My son's teachers know better than to spring a last-minute change in routine on him.
My son is a very picky eater, and tends to get very upset if there isn't something he wants to eat where we go. Right now before our upcoming trip in 22 days, I've been getting him ready for it, by telling him where we will be eating. And not to get upset if there isn't anything on the menu he wants, but rather to ask the server politely and sweetly if he can get what he would rather have. I've heard Disney restaurants are very accomodating when it comes to what the kids would rather be eating. He is excited about our trip, and knows that he's got to be flexible. It's a difficult thing for him to do, but he does try and eventually he'll get it.
I do have to say that I did get a GAC card -- FOR ME. I am deaf, and when we went last, I was still able to use a hearing aid. The GAC card bascially lets the CM know that I would need the use of a handheld captioning device or the use of closed captioning or a seat where I deem suitable for me to enjoy a program/ride. (Front row is not always best for me.) I was even given the script to follow along in for the Great Movie Ride. Now I have a cochlear implant (after I lost the rest of my residual hearing) and I'll be getting the GAC card again.
ADHD kids can enjoy Disney without any problems or extra assistance. It just takes patience and lots of preparation! I wish you much luck and lots of pixie dust.