From what I know long tehrm effect vary greatly from person to person, it can be very mild symptoms to severe. Focus on how is doing now, and that can rule out thing such as severe mental retardation and the like.
"The diagnosis of PMG is merely descriptive and is not a disease in itself, nor does it describe the underlying etiology or cause of the brain malformation.
Polymicrogyria may in fact be just one piece of a syndrome of developmental abnormalities, because children born with it may suffer from a wide spectrum of other problems, including: global developmental disabilities, mild to severe mental retardation, motor dysfunctions including speech and swallowing problems, respiratory problems, and seizures. Though it is difficult to make a predictable prognosis for children with the diagnosis of PMG, there are some generalized clinical findings according to the areas of the brain that are affected."