Hopefully the guys at Concord will be able to adequately fit it to your car; they managed to do ok with mine, and it wasn't perfect to begin with.
Did Dave at any point offer you a partial refund? I got $120.00 back when they botched the paint job- and while it wasn't enough for a respray, it was an ok result considering the part wasn't very visually prominent when installed, and looked 'black enough'.
FWIW, this thread is the 3rd link listed when searching for 'Talon diffuser' on Google:
Perhaps Talon Body Styling shouldn't have sold us their Talon Lancer diffuser bumper rear body kit part ... or whatever random combination of words you want to call it