If you want it to growl, put a different motor in it...lol.
Seriously, the 4 cyl just doesnt have any "growl" in it, besides, it is designed to run better with backpressure from a stock exhaust.
Sure, everybody wants their machine to sound cool, but a 2.5 is what it is. The "tin can sound" that you have might be coming from a worn out cat.
You can get a flowmaster muffler to give you a little bit of tone, and still keep good back pressure. My ideas are always form over style. I want it to run at its best, then worry about the rest.
Just my .02