A quasi-experimental study of graphing calculator use in grades 7-10 of one German state showed average performance increases were above the expected increase by all participating classes.  Average gains were: Class 7: ~12%; Class 8: ~6%; Class 9: ~10%, Class 10: ~12%

Bruder, R. (2008). TIM – A two-year Model test on the calculator use from class 7 and 9. The Joint Meeting of the 32nd Conference of the International Group for the Psychology of Mathematics Education, and the XX North American Chapter. O. Figueras and A. Sepúlveda. Morelia, Michoacán, México, PME