Wildcard virtual server for outbound and one to one SNAT problem

Sounds logical as I believe the VS is hit before the separate SNAT is done. And as the VS has automap, the separate SNAT never gets hit because the server has already been SNAT'ed to the floating IP of the BigIP.

You could work around this with an iRule on the VS. A nice simple one such as

when CLIENT_ACCEPTED {
    if { [IP:addr [IP::client_addr] equals "172.16.12.40"] } {
      snat 204.8.131.252
      return
   }
  snat automap
}

And remove the automap from the VS itself (Because the iRule does it for you. You could play with the iRule a bit and get it to use a datagroup as well for the matching... make it a bit more flexible when you want to change the IP's...