Since I put my Plextor Premium into an MaPower KC51U2G USB 2.0 case, it shows up as “Plextor Premium-U” in the device manager aswell as in the Plextools software.
Do you think should I flash it with the Premium-U firmware?
Can there be any advantage?
Greetings and Thanks in advance! 
That is completely normal. You do not have to flash your drive with the firmware for the external drive since this version is the same as the internal firmware. Plextor external devices are the exact same drives as the internal versions, down the the installed firmware. They do however identify themselves as a different drive when installed externally (the “U” addition). So, no worries, leave it alone and only install a newer firmware version if you need to (because a newer version is available for example).
They do not identify themselves as something different, at least not as to what I have seen. Only PlexTools adds those letters to the drive names. If you do an SCSI_INQUIRY on those drives, there is no U or whatever. You can see that, for example, when running PxScan on a “real” external Plextor drive.