IMO turret, or in general sub-construct, should be made using the same tools we have for main construct (voxels and modules). If it's turn speed is limited by mass, I don't see any major balance problems. It's of course a bit more complicated for devs, as they need to sync several constructs together.
But I don't really like FtD way of connecting hundreds of parts in a search for viable combination. I'd rather have simple list of sliders where you choose weapon statistics (damage per shoot, rof, initial velocity, acceleration, agility, guidance, burst duration/recharge, accuracy, ect.) and the game calculate cost (weight, size, energy/ammo usage, material cost, ect.) and shape.
Graphics-wise that's just two or three parts to switch models/scale (barrel, body, ew. base if not spinally mounted). Balance-wise IMO it's easier to tweak single formula, than 30 different weapons.