Time is a critical element of the GMAT. I know that personally, if I'm allowed to work at a leisurely pace, I can correctly answer almost every single quantitative question. Given the time to slowly and carefully perform all calculations long-form, the time to test each answer in an unfamiliar equation, the chance to graph or draw each example, 95-99% accuracy is not very difficult. But under time pressure, it becomes much harder. I think that most people are ecstatic if they can save a few mines on early questions so that they could spend them on harder questions later on. Based on practice tests, and extra 5 minutes would be awesome, and an extra 10 minutes would certainly result in a better score for me. But, everyone gets the same time, and everyone has to do all they can within that parameter.
Now, if you just don't even have a clue with how to proceed, or 'reverse engineer' a problem, the all the time in the world won't make a difference. For the verbal section, extra time would make a world of difference to many people, especially with RC.
I know that some people are given time allowences based on their disabilities. I don't want to sound harsh or callous, but even given a medical explanation I'm not convinced that anyone deserves extra time. The fact of the matter is, that if someone cannot compete on an even playing field when taking the GMAT, they can't really be expected to compete in business school, or when they take a real job. When you work till 2 AM and get 3 hours of sleep for a week straight at your I-banking job, nobody is going to make extra time for you so you can get your job done.
I'm probably going to take heat for saying that, but I'm just saying what I know to be true.