Well, IMHO there is no build in artificial speed limit in the USB/ATAPI-bridge.
My experience since my first external USB2.0 drive enclosure is that the old enclosures had a weak performance, 16x DVD was not possible, some chips had even problems with 8x DVD. The PX-708 read DVD-media at 12x, I hope the used bridgechip is strong enough for that
In general the enclosure is fast enough for every CD-RW-drive, the DVD-speed should be tested with CD Speed or something.
I don´t know the inside of this case, so I can´t answer this question. I can only say the newer optical drives are shorter than the old drives, maybe the cables are to short for newer drives