Proc chances are misleading as there is a hidden factor that normalizes the number of procs over a period of time to be the same between at least dual cannons and dual heavy cannons and possibly other types.
One thing to remember, all damage weapons and skills get buffed by tac abilties. And this buffing creates that makes the damage so impressive, even if the base damage is just a little better of cannons compared to beams. The different translates into a much larger difference once the buffs get their part. And then you have the crits on top of that.