This is about as specific as it gets (and I've emailed MacAlly about it as well), so I apologize in advance...
I have a Samsung 840 Evo 500GB in a MacAlly PHR-S250U3B enclosure (USB 3.0 and FireWire 800) and the performance of the disk, via USB 3.0, is significantly slower than I experienced with my previous SanDisk Extreme 240GB drive. As I understand it, the JMicron chipset in the MacAlly enclosure will link with a SATA disk at up to SATA II speeds (3.0Gbps), which is fine.
Previously, the SanDisk would benchmark at roughly the 3.0Gbps limit (275MBps or so, accounting for overhead) using the Blackmagic tool. The Samsung Evo is measuring about half that speed (around 130MBps). It would appear that the disk is linking at 1.5Gbps, not 3.0Gbps.
I found a thread (http://forums.macrumors.com/showthread.php?t=1641155
) that shows the Blackmagic tool getting 300MBps write on an Evo, so the issue should not be the SSD itself performing badly with compressed data. That disk was the 750GB model, and the specs for it and the 500GB model are essentially identical.
If anyone has experience with this sort of setup, any guidance you could provide would be very helpful. Thanks!