I received the same enclosure and it works flawlessly. However, I noticed the same phenomenon that the speed is locked to 10x sometimes. It happens when my "USB 2.0 card reader" is connected. It does not matter if I use the reader: As soon as it is connected, the speed of my burner is locked to 10x max. I am using an USB 2.0 PCI card with Via chipset.
In addition, I am using a PCMCIA USB 2.0 card in my old notebook. The highest speed I can reach is 8x. As soon as I connect my external hdd, the speed is locked to 6x Again, it does not matter if I access the hdd or not: As soon as it is connected and switched on, the speed is locked to 6x.
Conclusion: Since all USB PCMCIA cards are using the same chipset (NEC), I guess that it will not work.