Somewhat disagree with the metrics. I agree BC and Destroyers are not noticeable once on the battlefield. Destroyers feel like a big frigate.
They should have longer range (Battle cruisers use Ion turrets only in very short range) and be tankier than now (Destroyers feel really weak, like a frigate).
However the speed at which they kill frigates shouldn't be changed much.
The big issue with the game is TTK of Fighters and Corvettes. Every thing kills them nearly instantly. Fighters should be a counter to BC and Destroyers as they are fast moving targets for their big guns, but they can be killed easily by these behemoths. Right now, the only thing strong enough to counter BC and Destroyers are frigates swarms or the equivalent unit. Reducing TTK on frigates without bringing a proper counter in would break the game, IMHO.