Well, the CB gets to make their own rules. I am generally comfortable with the use of the WISC/WAIS PSI to measure processing speed for written tasks, but I grant you your point on sprinting versus sustained distance running is fair. There are other, more direct, computer-administered measures of sustained attention, such as the Connors CPT, IVA, and TOVA, but it is highly doubtful that your school has access to them, as they are a bit pricey, and typically owned only by hospitals or some private neuropsychologists. Schools don't have the funds for the license, or sufficient utility for them on a day-to-day basis.
At one point, ADHD could sometimes be used as justification for 100% extended time, but the research on the use of extended time for most persons with ADHD hasn't borne out substantial benefit. The best aggregations of research have recommended 25-50% extended time for ADHD, as more time actually causes a significant fraction of test-takers with ADHD to decline in performance (postulated to be because a little time pressure helps some people focus better). And, of course, the CB has moved over the past year to a greater focus on tests of processing speed and fluency as documentation for extended time.
On the WIAT, neither the oral discourse comprehension or sentence repetition component tasks is likely to be accepted by CB as documentation for extended time, since both of them have more to do with accommodations such as having oral directions repeated and rephrased, or chunked. Or accompanied by visual supports. They do, however, have a great deal to do with ADHD-primarily inattentive type.
I know you have a system of accommodations that works for you right now, and the obvious solution is to want to transfer that directly over to all other high-stakes tasks, but you might want to prepare for the possibility that this is not going to be the case. Some other strategies that you may wish to consider include:
1. create mini-races or targets for yourself, to take advantage of that fact that you can "sprint". So instead of looking at the AP exam as multiple hours of sustained attention, conceptualize it as a series of one-page mini-tests. So, for example, part I of the AP Physics exam consists of 50 multiple choice items in 80 minutes. Instead of thinking of it as 80 minutes of nonstop work, think of it as 4 20-minute intervals. In each 20-minute interval, you might target completion of 12 items. Spend 18 minutes of focused time, trying to get as close to 12 items complete as possible, and then give your brain a 2 minute break. Or use 8 10-minute intervals: 9 minutes of focus, followed by 1 minute of break, working on 5-6 items at a time. I'd suggest trying this on a few practice tests, and seeing how it works for you, once you get used to the rhythm of it. You won't be able to use an audible timer in the real test session, of course, but you can use a watch, and just mark the fifth item with the target time. Use the break minute to stretch quietly in your seat, practice breathing, shake out your hands, do progressive relaxation through your body (to release tension), or whatever nondisruptive activity you can do in your seat that helps you refresh yourself. And then dive back in for another focused period. (Just remembered you did get approved for time-and-half, so you would actually only have to target 4 problems for each nine minutes.)
2. Take a lot of practice tests, to increase your familiarity with the format of the test and the item types. Then you can at least minimize the amount of time you spend trying to understand what the question is asking for. Test-taking is a skill, just like the nominal content of the test is a skill. More practice and familiarity with it translates to greater speed and fluency.
3. It might be worthwhile to take a test-prep course, mainly because of the access to practice tests, and because sometimes they can help you learn to recognize the easy/fast questions, so you can fly through the whole test with those first, and then go back and spend the remaining time on the more time-consuming ones. A point is a point, so learning some of these test-taking strategies to make testing more efficient can be helpful.
How did you do on the essay section of the WIAT? That's timed, but for longer than 1-2 minutes.
