Jeff,
The y pipe is the same size and fitment as OEM, the Pre cats are removed and you get a nice smooth air flow in the pipes. The highflow cat removes the back OEM cat leaving 1, this is all bolt in and all the 02 sensor spots are kept. An exhaust shop will not charge you much if you have them install the stuff....
there have been dyno reports of 12+ RWHP...
The MPG increase is there, but most people cant keep their foot off the floor once they are installed..... LOL
Todd Z