i was reading a post earlier, which stated that it may be possible to gain up to a 5db gain from adding a port to a box. i find this very hard to believe, as one of the fundamental principles of physics states that you cannot create or destroy energy. since the extra volume from a port comes from the rear wave of the sub, there is no way it can be more than adding another sub. adding another sub gives a +3db increase in volume, adding a port gives UP TO +3db at the tuned frequency. in practice, it wouldn't even be +3db, as no system is 100% efficient.
Your right. Physics does state that you cannot create or destroy energy, but you can reduce the limitations or hindrance upon said energy. Please consider that many of these experiments (tests) are conducted under test lab conditions where OPTIMAL CONDITIONS exist. Also consider, "at what level of distortion was the test finalized?" Adding a port with a large enough diameter as to reduce air flow restrictions just enough to allow the driver to run at optimal efficiency in relation the increase in box volume. This will also reduce the amount of wattage need to run the driver. Also consider the surrounding enclosure (the car, trunk) that further limits sound wave expansion and distance. Theoretically a ported box can yield a 70% or greater increase of efficiency to energy usage by releasing the pickle within. Also, adjusting the surrounding enclosure to enhance sound pressures increases sound wave efficiency. So you can see that, although no system is 100% efficient, the effect can be increased by also adjusting the surrounding factors.
actually adding a second sub gives +3dB, as does going from a smaller sealed box to a larger vented box. going from sealed to bandpass will increase SPL by as much as +12dB with some designs. The box affects efficiency at the tuned frequency as you get closer to the sub's Fs Remember, impedance varies constantly in practical application, and thus, so does the actual amount of power the sub is getting/using.
as for vehicle effects, keep things like transfer point, and cabin gain in mind. these factors also affect efficiency to varying degrees.