NWEA currently does not recommend switching tests during the school year, but it does recommend that students being instructed in grade 6 math (which, presumably, would be the case for your DC next year) should be assessed on the 6+ from day one. So it seems likely that the school would appropriately test him on the 6+ test next year for both math and reading. That may allow for a little more spread, as there are more advanced items on that test.
You might find this document that aligns RIT subscores to Khan Academy math content interesting for home enrichment or school advocacy purposes:
https://cdn.nwea.org/docs/MAP+Growth+Grades+2-5+to+Khan+Academy.pdfNotice also that the learning standards go up to the seventh grade in some topics, but top out at RIT scores in the upper 220s.
