First none of Seagate thunderbolt adapters come with a cable.
Here are the speed results I obtained 3tb portable hard drive. I slid it into the seagate thunderbolt adapter. It fits very snug. I backed up some video files to the drive.
Speed test results are:
Reads in/second: 78
Reads out/sec: 86

I also have the seagate backup portable hard drive Thunderbolt adapter in 1tb and the speeds were slightly slower with the portable version.

I plugged in a Lacie 2tb hard drive into the port on the back of the adapter (daisy chain) and this feature worked well with no loss in speed during transfer test.

Also a problem surfaced that was not expected and not quite sure what is causing it yet. The hard drive would unmount itself after being connected for many hours. Thought it was the adapter at first, but think its the seagate plus drive plugged into it since the lacie thunderbolt drive that is plugged into the back of the adapter stays connected. Will contact seagate to see if they reply, and if others are having an issue and will report my findings.

Would have rated a star higher if seagate would include some sort of Thunderbolt cable.