For the price of a D16Z6 its cheaper than a couple of bolt ons for your A6, and you'll never see a 17hp increase for the few hundred dollars that a Z6 costs..
I don't understand why people buy intake, header, exhaust, cam, adjustable cam gears, etc. for an A6 when for the same price you could have just bought a Z6 or a ZC and it would be probably cheaper, and you'd have more HP from the get go.