As damage increases, firing arc decreases. Therefore, Dual Heavy Cannons, having high damage, should have a tight arc. Beams, having a wide arc, have lower damage.
This means that overall energy efficiency should remain the same. The problem is it isn't. Cannons are vastly more efficient (as an aside, ground weapons are much the same; beams do far less damage than bolts for no real advantage...).
The basic premise is that as you add something, you have to take away something equivalent to maintain balance. I don't see that here.
(and to put it in perspective... My Klingon character is max level, but is still flying a T3 D7 cruiser. With cannons, he can destroy a Tholian Orb Weaver in the Azure rescue mission in one pass, a matter of seconds. My Starfleet character in a T5 Excelsior armed with cannons, barely brings down the shields of the same Tholian ship, and takes several minutes to destroy it - exacerbated by the endless repeated disables the Tholians spam)...)
Once, I was simply called Mojo. Now, I'm forced into a new name, but don't be fooled, I'm the original STO Mojo!