My kid is also in private but they do use MAP. I don't know if the scores are particularly useful, though the test itself is said to be well designed. I didn't find the Lexile range generated by the MAP reading test results to be terribly useful.
I think that once you more than a few grade levels above the child's current grade, cultural knowledge and real world experience, rather than text processing skills, are the limiting factors in reading comprehension. I guess the important thing is that your child's text processing ability is sufficient to handle material in any other subjects that he or she may be accelerated in (e.g., science or social studies texts at higher grade levels).
Other than that, I think your child is "keeping up" if he or she enjoys reading and doesn't shy away from challenging material (as opposed to boring material), but that's just me.
