As someone who does a fair bit of escort pvp, I'd suggest that you'd be best off ditching the dual beam bank completely, and picking up a second copy of APO1 for additional maneuverability boost & tractor resist. Yes, an all-cannon layout does waste the ensign slot, which is why I don't fly my Defiant anymore in pvp.
I would recommend against Beam Overload 1. Coupled with the power drain incurred, it's not that effective as a damage buff. I'd recommend something like HYT1 or Beam Target Subsystem: Engines there. (I say Engines because even if you don't disable anything, the enemy will get slower, thus lose defense).
I would probably also decide to focus on either Beam Overload III with a DBB or High YIeld Torpedoes with Quantum Torpedoes, and not mix both. Your spike advantage is not that big to be worth the loss of steady DPS. That would be the second build then.