I have found that moderate driving after adding intake and exhaust in most cars (haven't had time to do it to my tC yet) will actually improve gas mileage. However, if you really like the sound of your intake and/or exhaust you are going to be screwing it down a lot more so gas mileage will suffer. At first, yes, it is going to hurt because you are going to be getting on it all the time and enjoying the sound, after a couple of weeks when the newness wears off, you get used to it, and your driving returns to normal your gas mileage will improve.
In the whole intake and exhaust cycle of a 4-cycle engine, how does adding a tuned muffler from the axle back decrease the amount of fuel used during the intake cycle? It really doesn't.
Now, that being said... A poorly designed and very restrictive exhaust system can certainly have an effect on the efficiency of an engine. In this case however, the tC already has a fairly well designed exhaust system, and adding a tuned muffler will change the sound of the exhaust, but you are not talking enough of a difference to have a significant impact on the power or fuel economy.
That is why I would invest my hard earned dollars toward a cat-back of some kind...I have learned my lesson before not to mess with axle backs because all they really do is add sound so it sounds faster...going with a cat-back can and has been proven to increase milage and power.