If you use an eSATA interface/adapter and assuming the SSD was just a SATA drive, then in that scenario, there should be no loss in performance.
The problem with SSDs is that you want them routinely powered on to refresh their nand cells, or you could run into data retention issues. By making them an external drive, you run the risk of forgetting about the data, and leaving the drive in some storage bin and after a couple years or more, you go back to it and the data is no good.