Though always curious about such things, I don't see a lot of benefit unless you are a bit shy of a score needed for a service. If PRI was lower and you had those (18,19,19) ceilings on VCI and a GAI of 137 and you were trying to get into DYS, then extended norms seems like a really good idea.
I think also the lower the age of testing, the more likely the extended norms will have a significant effect. Because the reason why there is room in the raw scores to be extended is because the test includes questions needed for higher ages.
