The book says you get a +1 or +2 bonus for high TL with weapons... Is this bonus per weapon, per turret, per battery, or just after everything else is calculated?
The bonus is for the overall USP of the weapon system.
For example, you install a triple beam laser turret, which normally has USP:2, at TL13 it would have USP:3.
In theory this is per battery. That is if you group the three beam weapons into three one weapon battery, each with a USP of 1, the battery has a USP:2, and you have three of them.