Thanks a lot for your answers!

I fuuly agree with fact, that plasma damage insn't so good. I can change damage type of my weapons.

BUT, I don't want to change dual heavy cannons. I use RCS ENG console which improve my turn rate and from my point o view is working well. Do you think that when I change my weapons type to phaser beams than will be my DPS higher?

That's why I have question: Why can Dreadnought load cannons?