Properly built CAIs use lengths and diameters of intake tubing that ensure maximum airflow into the cylinders across the entire rpm range of the engine ( not just in some RPM bands). Placing the inlet outside of the engine bay, while probably allowing cooler air to enter, may not actually increase HP at all due to the " de-tune" that it may cause because of the physical properties ( other than temperature) of the air that it is providing. Similarly, while the inlet for some systems may actually be in the engine bay, such systems may be engineered ( shorter length intake pipes, etc.) so as to be "tuned" to allow the best phyical properties of the intake air ( quality and quantity) to fill the cylinders better across the entire range.

Bottom line is that if you are choosing a CAI based on whether the intake is outside the engine bay or not, you may not be getting the best system for your needs. It depends on things in addition to the temperature of the air.

Anyone wanting to understand CAI and the dynamics of airflow into an engine need only read up on the principles of RAM induction as a good example of how air intake properties relate to increased horse power.