The reason why Petro is regarded as so strong is because:
50mm deck and upper hull - Nothing overmatches it and most cruiser HE shatters. This makes it extremely resistant to most shells.
Sits very low in the water - Citadel is hard to hit even broadside because it's "The First Submarine".
Advanced Penetration Angles on AP - The guns have Stalingrad pen angles and extremely high velocity.
Thin top-down profile makes it extra-resistant to most CV's.
All this makes the Petro just too damn useful to NOT take in competitive modes where things get hyper-optimized.
I realize that a half-second reload nerf seems low. You have to understand that things like the Petro have to be balanced delicately and carefully because the top players / clans in the game will determine whether or not something is broken as they optimize. No joke, there will come a reload time when the top clans go, "yeah it has the armor, but the DPM isn't worth it anymore". That line has to be approached gingerly rather than leap-frogged.