When my kids have taken the MAP or WJ-III their GE scores do match up well with their ITBS scores.
Without trying to minimize your level of experience, I think that at most tends to show a correlation with how the GEs may be arrived at, not that they would actually score at or near the 50th percentile on the test for the grade where they are reported to be around the 50th percentile. And your particular kids might score that way on an 8th or 9th grade test, but I still doubt (without more evidence) that the average score on those tests for children with those GEs would come out that way.
ETA: Okay, that was boneheaded. Rabble rabble small sample size.
