WD TV SMP USB 2 Speed?

Is the USB 2 on the SMP hi-speed (more like 320 mbps - assuming a hi-speed 2.0 device is used) or regular 2.0 (more like 12 mbps)? 

Your terms are incorrect.

USB 1.1 is 12 mbit / sec.

USB 2.0 is 480 mbit / sec.

USB 3.0 “Super Speed” is 5.0 Gbit/sec

The WD devices have USB 2.0 interfaces.

It is more like USB 2 but far from the theoretically possible 480 Mbit/s. My estimate is about 25 Mb/s that is about 200 Mbit/s.

480 Mb/sec is the clock speed for the USB hardware, not the throughput of the device.  There is buffering, buffer size, ACK/NAK communications, chipsets that are slower than the maximum, rotational speed and latency of the hard drive, and so on …  the actual data transfer rate is depedent on many more factors than the clock rate.