You won't get 16x burns with this (or any other) enclosure and your BenQ 1655, unless BenQ releases a firmware capable of running UDMA 4.
If your system is up to it, you will be able to read and write at 12x.
Maybe, just maybe, you'll be able to read at 16x with FireWire, but probably not with USB.
To get optimal performance of your BenQ in a PL-3507 enclosure, you should make sure the enclosure has the latest firmware (available here).
In general FireWire has a slight performance edge, and it uses less CPU than USB. Whether you'll notice any actual difference is difficult to say.