Most (if not all) of the buffs add a percentage to the production of a workyard, so its dangerous to say something is doubled or trebled. As far as I am aware if the base production is 1 then a +100% buff would make it 2 and the application of +100% zone buff would make it 3.
It will be interesting to see if the barracks add on will add to its base production (So it will become base [including addon] 1.5, +100% = 3, +100% = 4.5) or just act in the same way as a buff (base 1, + addon = 1.5, +100% = 2.5, +100% = 3.5)
.. and yes those contributions look horrendous, 400 odd besiegers
Well its just come back up so we shall find out very soon .......................