I realize I'm jolting an old thread back to life, but here goes.
It's interesting that some people consider it to be junk and dismiss it as a gimmick. When you look at the science of it, you begin to see why it was installed on a guitar. After all, bass cabinets most often have ports. All ports are, are sound holes. And some ports, like those in my ACME Low B2, are specifically set up to increase the tonal response and handling in certain frequencies. If you've ever popped open a Low B2 cab, you'll see that the two ports in there are pretty deep. So, porting of a bass enclosure DOES indeed work. The science says it does.
What the the OP needs to consider is, what kind of wood is the guitar made out of? Is it a bright wood, or is it a dark wood? Is it a bass heavy in dense lower overtones, or is it really bright? Does the OP play with a pick? On what kind of strings? All of these are important choices a musician takes into account due to how, when added altogether, they sound on any given instrument.
Will it be magically as loud as an upright? No way. Will it be louder, or better sounding? Quite possibly, it depends on your instrument's wood and body size, really.
The one thing that does concern me is that port science is just that. Science. A lot of math goes into creating a port for a specific size and tonal quality needed for a given enclosure. And the O-Port is a one-size fits all kind of deal, that is intended for the dramatically smaller acoustic guitar. So, it would stand to reason that, for the average cubic volume of an acoustic bass guitar, you've probably got the wrong math going with an O-Port, and it may not help you at all, or at the very least, it may not help you to the degree that a properly measured port would help you.
Just some thoughts I had while reading this thread and watching the videos of them on YouTube, where it had a dramatic increase in fullness and clarity for some guitars, and not so much for others. And that I could find, there were no ABG videos. I may just order one and see how it sounds and make a video.