Add SSD to iMac: Thunderbolt or internal? Is there any thunderbolt SSD which can be as fast or at least not much slower than internal one from OWC? I want to use is as system disk. It will be good to have two thunderbolt ports on this thing to daisy chain display.

It will be also a big bonus if I can buy it in Europe (because of specifics of organization where I am working).
Thunderbolt is 10 Gb/s. Internal ones use SATA II or III at 3 or 6 Gb/s, respectively.

However the Thunderbolt ones use SATA III internally. So you won't see any difference.