Will TRD Exhaust hurt my Miles Per Gallon?

No. Only your lead foot will.

Any idea on the amount of improvement if my foot does not get heavier?

im guessin around 1 - 2 mile extra?

Zero

should b at least one, cuz easier air flow

Not necessarily. Think about it.

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.

i love those deep tone sound!