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It depends, if you have other mods (especially ones that increase the air flow, like a charger, cold air intake ext.) then it does make a difference. Think about it this way: the more air, the more fuel the car pumps,which means the bigger the explosion which means the more horsepower.
A cold air intake makes the air entering the engine more dense, when things get cold they shrink (pun intended) which means more air entering the engine. Getting a bigger t/b and plenum opens the intake up to take more air. If you are planning to do other mods later deffinitally do it. It is easy to install and helps the airflow. Cheap horsepower is good. Before I supercharged my 04 GT I installed a cold air intake and it was ok, but it felt better when the t/b and plenum was installed.