OP, I have been running the Mirror Qin since I hit 40 and I have yet to enjoy anything else as much as I do that ship. If you like it stick with it, a non fleet ship will do you just fine in endgame.
Personally I have had very good luck with the Fleet Patrol Escort. I prefer the universal ensign over the dedicated Tac ensign, gives some flexibility for survivability, that the Fleet Patrol offers over the other Fleet options. I run it with MACO MK XII and the Adapted Borg tech set and tend to do well in ESTFs.
I ran the standard patrol escort until the start of 2013. Its still very viable, and a tough ship with the engineering lean. I ran 150+ ESTFs with it. I can only speak for PVE though.
OP, what is your fourth aft weapon? What about running turrets in the back to help deal with incoming target-able torps? Other than that, I think this is going to be getting tested this weekend on a few ships. Thanks for Sharing the build!