If you ask me, the totally-different Terran Empire/UEP starships and WIPs may look nice, but they would never ever exist in the Mirror Universe still, in my opinion. If you want a true Mirror Universe with the Empire existing in the 24th century, events and ships must be similar, albeit not identical. Perhaps basically the same, yet with differences that make it dark and evil, much like Diane Duane's I.S.S. Ent-D description or Atolm's "re-imagined" I.S.S. Enterprise. But I'm probably repeating what I had said before in the thread.
Whilst it's my opinion of an idealistic "Mirror Universe" to mirror the Trek universe in a dark way in the 23rd and 24th cenuries, I'm sure not many would agree with that, nor would many limit themselves to the Physics dealing loss/addition of mass and energy (as it can't be created or destroyed) like few had. It wouldn't be interesting for space battles between good and evil Terrans. *shrug* So if we go by that, some of the designs recently displayed/linked here are alright.
As well as a Mirror Universe episode on Enterprise... It's nice, but I've long since abandoned that series. *shrug* Maybe I will watch it when it comes to TV this year.