Not necessarily. One of the reasons Borg ships are monsters is because they are being piloted by the collective crew consciousness. That is also why they are able to repair and regenerate quickly. A "captured" Borg ship piloted by normal Starfleet officers would not be operating anywhere near the same level as a ship piloted by actual Borg.
Unless they were piloted by liberated borg. Borg doffs, too, maybe?
You must be forgetting that the ships already available to us can solo Borg cubes.
You're right, but a captured borg ship with hybrid borgo/starfleet tech could potentially be the strongest borgo/Fed ship (if we consider it a hybrid class like all other fed ships that have borg systems) yet, capable of taking on a tactical cube by itself.
My vote would be captured Fed/KDF ships. They could have scaring and perhaps graffiti on them in the case of Klingons.
I would like to point out at no point did they say lock boxes were going to only have starships as the big surprise. They could easily put shuttles or special weapons/armor or exotic bridge officers with special abilities or models like a Tholian for example. Or unique race unlocks for account.
In fact given nature of next FE series, I would think maybe next lockbox could be captured Tholian ship.