I'm reading How the Gifted Brain Learns right now as well. I've found it to have a lot of good info on twice exceptionality and underachievement as well as ways to reverse patterns of underachievement. You might want to check that one out as well as the Misdiagnosis book.
In my dd's instance, I think that she does just tick more slowly. For instance, when we are volunteering at our local humane society, I can clean 2/3 of the cat cages in the time that she does well less than 1/3 of them. Although she does have some perfectionism issues, I really don't think that they play out in regard to litter boxes and cat food.
If your ds did well on the block design part of the PRI, which is also timed, and seems to be more impacted by perfectionism than true slower speed, it may be something that he will improve with more appropriately challenging work. My dd was in the 25th percentile for the block design test and the 99th & 99.9th for the other two tests on PRI, which again showed the slower speed there. I do think that some of her anxiety issues related to being timed came into play on that test as well, though.
