I think you would be more likely to see an obvious leveling off by looking at math growth.
Also, a lack of reading growth might be due to a lack of interest in higher level materials for a few years. For example, a 6 year old who is reading at a middle school level might stay there for several years simply because the most interesting books to them are written at the 4th-6th grade level. In fact, as far as I have been able to tell, *most* modern children's books are written at this level.
Of course, more challenging materials could be introduced in the form of classic children's literature, which generally seems to be written at a higher level for the same target audience.
Just stuff I've thought about over the years. Of course, I didn't answer your question and you probably already knew all of this anyway. Good luck with your research!