I have heard anecdotally from a number of teacher acquaintances that thick adults consistently start as thick kids. It's possible that the apparently-thick 5yo will turn out to be a genius, and that the apparently-bright 5yo will turn out to be a doofus, but it's not the way to bet.
The other problem with educational snake-oil appears to be that it tends to be tested on bright kids - who would learn to read off cereal packets and street signs. So whatever woo you put them through, they'll do not too badly. The woo then fails when average or thick kids are put through it.
Most teachers don't want to take responsiblity for not turning dumb students into smart students. As a result they are no good source for knowing whether it's possible to turn dumb students into smart students.
Post by fellow LW reader Razib Khan, who many here probably know from the gnxp site or perhaps from his debate with Eliezer.