I just installed the Hawk Header,and it is great. The nice thing about the hawk header is the O2 sensor is on the collector where all four pipes come to gether, where as on the Calmini and Doug Thorley it only read from two pipes. Which mkight give a false reading and may make it run a little off the norm. The Hawk also dosent require a gasket where it connects to the rest of your exaust. It has a ball swiviel flange. I installed the Hawk Header, Catko Cat, Magnaflo Hi-flow Muffler and 2" pipe all the way around. It gained noticable difference. Hope that helps. There is also an article about the Hawk header on this website from last month i think.