Numbers from Early Childhood Education in Oakland, CA in the 60's and 70's too, I think, show the following:
1. Kids in the EC program learned to read and do arithmetic earlier and better than kids in the control groups.
2. By grade three there was no significant difference in group achievement.
I did the statistical analysis again from original data and got the same outcome; i.e. initial improvement followed by leveling off to equal academic achievement.