Hi there,
As a sonographer i can't tell you that i know a lot about autism, nor do i know a lot about the research and relation of autism and choroid plexus cysts. What i can tell you is that we see these cysts quite often at 20 week scans, which tend to subside by the 3rd trimester. As an isolated finding, there is usually no risk for genetic abnormality. The hands should also be checked to insure they full open. Clenched hands plexus cysts can also be a sign of (trisomy 18?) i believe.
So don't be overly worried. Have the baby scanned again in the third trimester to check on the cysts. You can always seek genetic counseling, or an amnio if the baby had other abnormalities.