You aren't going to see a full TB bandwidth with any current single SSD ... the SSD data rates simply aren't that fast. The apparent 350 MB/sec "cap" on the Seagate TB adapter is probably the actual speed of the enclosed SSD.
If you use a multi-drive TB enclosure, you can run 2 or 4 SSDs as a striped RAID-0 which will start to stress the TB interface speed. I use LaCie "Little Big Disk" Thunderbolt enclosures with 2 SSDs, and I also use a Promise J4 which holds 4 drives. With both of these Thunderbolt enclosures, the drives appear as separate drives to OS X and you use DiskUtility to RAID them together. Note: the Promise J4 requires a driver which precludes being able to boot OS X (or Windows) from the external drive, but I use them for data only, using the internal drive for booting.
-howard