Right this minute, I can't seem to put my finger on the article I know is right here on the site that discusses this. I know I've heard the Sisters discuss students' gaining up to 2 years growth. I'll keep looking . . . 
One thing I see in kids I work with that makes me smile even more, is that kids LOVE books and reading--and choose it over other activities. I especially love it when they say things like this, "What?? We can't have Daily 5 today because of the Holiday Program??" And truly mean it :).
Love for reading is the biggest payoff!!
Enjoy 