You can find used NPP catbacks or axlebacks for cheaper than what it would cost new on the GM parts sites. I myself got the axle back for around 500 as well but the flaps are always open. I haven't put a Mild2Wild switch on it or run vacuum. The sound is an improvement over the stock, however the volume of in cabin noise to me personally is a little too loud, especially on the highway. Of course it's subjective and people's tolerance of noise level in the cabin is up to them, but for me it does get annoying. Sounds great outside though when wide open throttle.
