Like an earlier poster mentioned, we've seen leveling off due to lack of appropriate instruction to be more likely in math than reading. Our oldest's MAPS RITs in 4th grade were in the 99th for reading and 97th for math at the start of the year. She spent a lot of that year tutoring other students and just coasting through.
However, she loves to read and still read a lot on her own. By the end of the year she was still in the 99th for reading and her RIT growth was about twice the expected growth while her math growth was less than half the expected growth and she was down to the 91st, I believe.
She is a child who is so far ahead in literacy (reading and writing) that her test scores would be beyond the 99th for 12th grade at this point, though, so unless she actually regressed, she'd still come out in the 99th even if she had no growth.
I am in no way an advocate of leaving gifted kids to their own devices to learn with no instruction. I do think, however, that a verbally gifted child who likes to read is more likely to continue making progress in the absence of appropriate instruction than is a mathematically gifted child with limited instruction. Math is harder to self instruct and few kids do math workbooks or read math textbooks for relaxation so they don't have as much of an opportunity to learn through osmosis.
