So, when the need arises the rules allow you to "...disable one of the ship's weapons..."

Once disabled you could redirect the power to another system, like the engine, adding 1 movement to the ship.

So.. my question is what is "one of the ship's weapons"?

If the vessel has just a single laser cannon, the answer is obvious. If the ship has something like "Blaster Cannon(2)", does the action disable both?

Some vessels have their weapons grouped, like a star destroyer...
Weapon : Turbolasers(60)
Weapon : Point Laser Cannon(40)
Weapon : Ion Cannon(60)
Weapon : Tractor Beam(10)

Would you have to disable the whole group to get enough power to re-direct? I would say yes. If you only had to disable a single gun out of a group, then the Star Destroyer would be the fastest ship in the galaxy if it disabled all its weapons. It could move 173 squares per action! Yikes.

What about disabling a portion of a group? Like, if the turbolasers are organized into 12 batteries, could you get any benefit from disabling a single battery?

Again I'd say no because of the potential for the Star Destroy to go so fast. I would allow the players to disable half of the weapons in two separate weapon systems and gain a benefit.

Thoughts?

Sounds good.
I might even go so far as to say that does not apply to capital ships at all. Disabling the weapon systems of such a large ship might not create enough extra thrust to make an appreciable difference.