If it works with both chips inside, why bother with getting a matching stick? If you are having problems - then yes get another similar stick. If not, just keep what you have,
The AMD APU is one of the few CPU's out there that benefit a lot more than usual from a dual channel set up.
And even if you did get a matching stick - I highly doubt your scores would go up much (if it did, it would be like less than 10%).