If each cargo, weapon, and ammo have a certain mass, and the more mass carried in the ship, the slower it's manueverability and accel/deaccel? This might give lighter (eg. blaster/taky) carrying ships a chance against Missle/Rocket boats and such... make the missles and rockets heavier (plus thier ammo)

Been discussed before. It's a good idea, but it would take a lot of calculations and put heavy ships at an even bigger disadvantage over light ones since they already have to carry a lot of weapons and cargo in addition to their initial mass. They're unmaneuverable enough already.

Well it is planned, we know that much. I doubt they would use the current agility of heavies as the empty loadout. I think the current agility would be used as one that has all the weapon ports filled and a handful of cargo. At least I hope. Anyway, implementing and balancing a bulk system like this would take a lot of time. It'll take some time.