From what ive seen, the stock HU puts out like 10-15 per speaker. for a total of 50-60w.
Adding a sub is definately a good way to get some more boom. I highly recommend getting a different headunit though, even a $120 kenwood or sony unit will get better sound than the blapunkt unit, and its easier to hook up a new amp/sub to a aftermarket headunit. Not to mention giving you aux inputs and other such fun things.