Here are the Thile-Small parameters for this woofer:
Fs 26.6 Hz
Qts 0.43
Vas 1.75 ft.
Xmax 0.17 in.
Recommended enclosure size 0.7 ft. sealed
With the Thiele-Small specifications it has, the Sony Xplod subwoofer should work well in a ported enclosure. There are a couple of reasons why the sealed enclosure would be recommended.
A) If a woofer works well in both a sealed and a ported enclosure, the sealed enclosure will be smaller but have a higher cutoff. In a car, obviously, space is a premium.
B) In a ported enclosure, there is a tendency for the woofer, near the tuning frequency, to travel all the way to the end of it's travel and to exhibit a clipped waveform, highly distorted. Special construction techniques, such as a Faraday ring or extended pole piece, are required to prevent this. Sony makes no mention of these construction features on it's website-it might or might not have them on the Xplod subwoofer.
Assuming it does have these features, the Sony Xplode will work well in a ported box of 1.25 cu ft, tuned to 25 Hz. That would require a port with a 3" diameter and a length of 22 inches. With such a small excursion, (.17"), the woofer would probably work well with a 2" port of 9" length.
Here are 2 plots with the Xplode subwoofer in a .7 cu ft sealed box and 1.25 cu ft ported box. Notice how much more output at 30 Hz there is in the larger, ported box.