I have used 1 dual Phaser bank on each of my science ships due to the turn rate of the science vessels they can be brought to bare on target quite easily and do more damage than phasers. not noticed any power issues but I only use one per ship?
I'm a Captain 9 (14 bars to go - woot). Currently I have 2 beams, a dual, and a torp up front, and 2 beams and a torp in the rear, in my cruiser. When I get my next ship at Admiral, I will either be putting in another dual in the back, or mines. Not sure which, but probably the dual. I find the plasma release skill as effective as mines.