1) There are probably not enough of these cases to get statistical power and there is the major potential confounder of the freeze-thaw itself. However if technology allows this to become more common, and the Flynn effect is still occurring, we'll start to have twins born decades or centuries apart to help us find out why.
2) Derek Parfit would have something to say to people who are hesitant to let their frozen embryos get implanted. Your genes are locked in a test tube, never able to experience happiness? Or raised by someone else and on net happy more than suffering, 20 years from now? If there's no chance you're going to use them, then #2 is a no brainer, if you think causing a hypothetical person to miss out on hypothetical happiness is immoral.
A frozen baby from the future. As it turns out, Flynn was only half-right: not just IQ rises over time, but also average birthweight, here up to 35 tons.