You're welcome. I'll note that the SB5 development team made attempts to regain some of that power test quality, using item sets, rather than straight x-in-a-row-incorrect ceiling rules. It also covers the range continuously from age 2 to adult, so that, hypothetically, a toddler could test into the mature adult range of difficulty. (Although I think most would be well overdue for a nap long before then!) Speed is still less of a factor on the 5 than on the Wechslers.
You are correct that deviation scores do not technically tell one anything about performance compared to older/higher grade students. This is one of the key reasons why the use of age/grade-equivalents is strongly discouraged. The test is designed to order an individual within their age (or grade) peers, not to compare them to populations in other ages (or grades).
Most systems use deviation IQ data as only one of the contributors to grade-skipping decisions. A model that might make a bit more sense might resemble that used by the home district of my youth, where early entrants to kindergarten underwent a complete battery of cognitive and academic achievement assessments, followed by two weeks of trial kindergarten in the summer, prior to being admitted to kindergarten for the school year. Of course, that's a bit labor intensive (and rather expensive), not to mention practical only if you happen to be a district (such as that one) with an unusually high number of applications for early entrance to K, such that a summer lab class can be formed.
