Definitely underpowers them. The Shadowcaster makes up for its relative lack of flexibility compared to turrets by having abilities that tie into having enemy ships in its mobile arc. The reduction in actions for all other turreted ships (which would have to spend actions to reposition their arcs) without anything to make up for the loss would throw the balance way off. Turrets in themselves aren't that powerful these days.
Thought about it? Yes. However, the problems as I see it with doing this:
1. It absolutely underpowers the ships, and their point values are now skewed way high. I couldn't even begin to guess at an appropriate cost reduction.
2. They would lose action economy big time. Right now, all three of the named Shadow Caster pilots, plus the title card and some of the upgrades give benefits for positioning of the mobile arc, which helps make up for the loss of action to reposition. How would the other ships make up this same gap?
3. You would then need to do this for all secondary turret weapons, and find a way to cost them appropriately and make up for action loss for repositioning.
A part of me wishes that they had done this with turrets from the start, but it seems too challenging to try this retroactively with how pervasive they are in the game.