We already have the voluntary Georgia Pre-K program, and here in N. Fulton county, at least, it is extremely popular. It is difficult to get space in the Pre-K of your choice if you don’t sign up very early. For instance, my sisterinlaw went to sign my nephew up at two locations near home, back in February. She only managed to get him on the waiting lists for both of them, she is spot #2 on her second choice school list, and spot #6 on the first choice. I believe that the families in this area are of a mind that enrolling the children in this program will give them a leg up on the other children in kindergarten, I know that is my SIL’s motivation.
The downside of the pre-k program is that once the children are enrolled, they must stay enrolled in school until they are graduated from high school, they attend school the same number of days and hours as the other public school children and are subject to the same rules as far as attendance (i.e. no more than three days per year absence without a doctor’s note), etc. A typical pre-k requires that the 4 year olds be at school at 7:00 AM until 2:00 PM M-F.
If you'll notice the Georgia State study referenced in the article, I'll bet that is the reason a lot of middle class families enroll their children - and I bet it's one reason the program seems to exascerbate the achievement gap.
I haven't read the study, but I'd be willing to bet that while the lower-level children improve, those at the top improve more.