The Mirror Universe ships are the exact same as their normal universe counter parts only the Ensign changes.
The Sovereign is exactly the same as the MU Sovereign except the MU has a Sci Ensign. All other stats are the same.
The ship you get at L40 is a T5. I don't know where the concept of it not being T5 came from. C-Store ships are generally considered T5+ but it's meant to come down to the better skill/build of the Player over who would win.
I suppose you could consider the 10 Console Fleet ships T5++. I'm thinking T6 will never come or simply upgrade a BOFF station.
The way PWE seems to want Cryptic to handle things, they will never release a new tier. They will play coy and just slowly increase the power of new c-store ships until all our old purchases become obsolete. Revamping PvP will help in this regard because the power advantage of the new T-5 ships will become more relevant.