Unfortunately, USB3.0 will be significantly slowed down by the internal SATA II port.
With a Samsung 840 Pro SSD and the housing at the USB3.0 port of iMac (Late 2012) I come in:
- SK-2500 to about 100 MB / sec. (For firmware update)
- SK-3500 to about 200 MB / sec.
One tip on the edge: If one does not stick the rubber feet directly on the screws, to get even later just fine on them (the screws) zoom.