We never sought extended time on tests through talent search for my dd who has processing speed issues although she has had a 504 plan since 4th grade. Our thought process was that it was beneficial for her to try taking tests that were too hard & with not enough time so she would hone her efficiency and test taking strategies as much as possible. She always did well, but I'm sure that the extra time would have helped had we been looking for her best performance.
In my dd's case, her WMI was in line with her PRI and VCI and processing speed as well as block design (which is the only other timed part of the WISC and part of the PRI index) were low avg - below your ds'. Since she's now in 10th grade, we are looking at whether she can get the time and a half on ACT & College Board tests that the pp mentioned. From what I've been told, it is nearly impossible to get more than time and a half. The OT who recently reevaluated her said that she'd essentially need to be missing her arms to get more time than that.
For College Board time accommodations you need to have: IQ test scores showing a discrepancy btwn timed & untimed pieces, achievement test scores showing the same, and a dx of some disability that shows why the discrepancy exists. Just showing that it is there isn't enough. You need to show why and, ideally, it should be something in the DSM from what I understand.
I'll send you a pm if you want once we get through the process of applying for accommodations and let you know if we were successful. FWIW, my dd's recent testing (achievement and OT dx had to be redone b/c they must be within two yrs of the application date) had huge discrepancies. For instance, math fluency on the WIAT was well below the 50th percentile, writing fluency was avg, and untimed pieces for math & reading were 99.9th. Her dx are SPD & dysgraphia plus dyspraxia from when she was younger.
