I hate to sound like the devil's advocate but I've been drinking and I think this is an important thread for people with a learning disability. I'm ADD and have done all the physc evals and such for many years and have always done well in school but horrible on standardize test because I can't finish.
I refused to take the GMAT with extra time because I know I'm not always going to get special treatment for my condition. When you get your first job out of business school they aren't going to feel sorry for you because you have a learning disability, you either can or can't do the work. I know there are many talented individuals who don't test well but I know if you are committed you can master the GMAT.
I got a 640 the first time I took the test with scores not much different from yours. I spend the next couple of months practicing for the test. I did a ton of practice test and prep work and was able to score a 710 my second time around.
If you want a better score I think you can do it, just commit yourself to doing better instead of writing an essay about how you can't do well. Don't have things handed to you because you can, overcome the odds and get that 700+ score despite your learning disability and prove to everyone you are a person who will overcome any odds to be successful (now that's an essay)