Ok, what it sounds like you're saying that you have a sealed enclosure for 2 10's, and you are wondering if you can port your current box. First off what type of subs are they, next you need to know the airspace requirments for those subs for a ported enclosure. Then measure the airspace of your current box and see if its enough to port. Most likely its not gonna be. Therefore you're either gonna have to build another one or have someone do it for you.
Ported enclosures do give you about a 5 db increase, and in a hatchback you will get more bass. You do lose some sound quality when you switch to a ported enclosure, plus they are less forgiving if you don't get them built perfect to specs.
If you're wanting to learn more about car audio check out the
http://www.caraudio.com forums.
Eric