One of my big hopes for Romulans is a chance to pilot one, and seeings as the Klingons get a tier 5 refit of the Br'el, I hope for similar for the BoP.
Facts about the TOS BoP that make it distinct ;
1) No battle warp drive.
In the original show they stated that BoP did not have Warp drive. However traveling at sublight everywhere would be very unrealistic. This statement has been retconned since to state that Romulan ships simply could not go into Warp on a whim. They cannot use their Warp drive in a battle, they need time to build up a warp field.
How does this equate to STO? We already cannot go into Warp in a battle. I would say that they get a penalty to Driver Coil. Refit version ignores this, using a more modern warp drive.
2) Cruiser ;
The BOP in Balance of Terror was a Cruiser.
3) Weapons ;
The TOS only had one fore weapon (Plasma Torpedo) and one aft weapon (it carried nuclear weapons, replace with Mines. They were deployed as mines in Balance of Terror.)
While under-weaponed seemingly, the BoP was still able to go toe-to-toe with a Constitution class ship of the era. This should be addressed. The Romulans did not get proper beam weaponry until they traded tech with the Klingons.
I suggest compensating by giving the ship two extra Tactical slots than an equivalent ship of its tier (for the basic version and the refits.) Give the refit an additional Fore weapon for beam weapons and a second Mine slot on the back. And give it Torpedo and Mine based skills inherent much like Science vessels get Sci skills.
The original version of the BoP was able to deploy the nuclear weapons while cloaked, along with debris to fake its death. The vessel should be able to use mines and mine based skills while cloaked, but not other weapons or skills. Unlike the Br'el, it doesn't have to decloak to do this. This makes it better in one way, but weaker in another.
Special Weapon ; Nuclear Warhead. Deployed as a mine. They also have a large blast radius and do Radiation damage as well as Kinetic and can knock out systems due to EMP.