I got one of the first cat back systems from Cork Sport. All I can say is was way too loud. I had a 2008 Speed3 with Cork Sports exhaust which was loud but the speed3 is a lot different than a CX-9.
I called Derrick at Cork Sport and asked him if the axel back would fit the stock mid pipe and he said yes. Long story short I am very happy with it. The full cat back does spool the turbo faster. I would go with the axle back and if you want more I will make you a deal on my mid-pipe that's sitting in my storage unit.
I think based on what speed3chris1 said, the midpipe is what provides the bulk of improvement in the system. I'm tempted to buy just the midpipe but I don't think CorkSport sells it separately and it would be too expensive and probably too much of a hassle for Chris to sell it to me here in Canada.