Adding a tuned port is a cheap way of adding thump (and distortion). A tuned port is more than a hole, but a tube of specific volume that will increase the volume of a specific frequency window.

If they are actually tuned reflex ports (and not just a hole), they could increase the low frequency energy and shift the low resonance frequency, but to truly work the enclosure volume would need to be enlarged. Adding a port to the existing 'enclosure' actually removes volume. So, it's counterproductive to just add a port without modifying the speaker chamber too. It might add thump, but your likely to just add distortion with no real overall increase in sound volume.

The stock mounting is a pseudo acoustic suspension design that gives a tighter more controlled bass that has a typically flatter response curve and lower distortion. Bass reflex (ported) designs sacrifice flat response to give more power to a lower freqency and more distortion. Whatever frequency the port is tuned for will usually add thumpiness for about a half an octave around that frequency.

Tuned port: adds thump and distortion, loses upper bass response, requires larger enclosure
Sealed: flatter response, lower distortion.

A better way to improve the stock enclosure is to stuff it with polyfill fiber (the stuff in most pillows). By breaking up standing waves it actually increases the effective volume of the enclosure increasing the power of the lower octave without adding the thumpy rattle and distortion of the port modifiction. Also, since the enclosure is by its nature flimsy, resonant, and leaky (unsealed walls of plastic trim and body panels) the polyfill will absorb some of the energy that would otherwise go into making those parts of the car rattle at higher volumes.