That is a custom jobbie. And not a great one either... its not symmetrical.
I know the one in the photo is custom and yeah it itsn't that great of a finish either... I just can't seem to figure out how it's made without splitting either a Mk6 GTI rear valance, chopping it down to fit the mk5 one and then cut out the holes from the mk6 bumper - melt it together on the mk5 bumper - sand it - fill it - prime etc etc etc
Would be a lot easier if it's possible to buy one instead hehe..