well ive just bought the 172 with ktec already on. Must say im impressed. The sound is a NICE sounds....very deep and not raspy like u hear all the chav racers and there bean cans out the back of the novas. I wouldn't go as far as saying it is quiet though either. If you give it a heavey right foot you will certainly here it! but with normal driving, you can have you stereo on volume 10 and hear it perfectly....or withold a conversation easily.
To put it into perspective, i drove 150 miles on saturday and it wasnt a noise that bothered me or was loud, and im usually a person that prefer silence.
hope this helps. Id certainly recommend it, sounds extremely NICE.