Hi there! Sorry I'm a little late, but thank you for using the mod!
This is actually an intriguing suggestion!
I have actually played around with various implementations of the mod using linked chests since they were added to the API, but there were several problems that stopped me from switching to them.
1.) The most UPS intensive part of this mod by far, surprisingly, is inventory checks. The invisible inserters are not managed at all by the LUA code (except pointing them in the right direction), and thus use factorio's native (and very optimized) implementation. Checking if a turret has a better provider available, checking if a chest can still provide the ammo the mod wants it to, etc, account for far more UPS usage.
2.) I found it difficult to use linked chests to mitigate these problems specifically, since most of the ways I tried still required me to check the turret and chest inventories to assign a provider to a consumer. Implementations using the linked chests used only marginally less UPS. Another reason for this fact has to do with #3:
3.) even with linked chests, there still has to be an entity that moves the ammo to the turrets. Because of the way factorio works, you can't link the turrets inventory directly to a linked chest. This means you either have to replace the turrets inventory programmatically (which is much more UPS intensive than the invisible inserters), or you have to have an inserter for each turret that reaches to the linked chest, essentially creating the same problem you had before.
Still, your suggestion has a lot of merit. If I allowed the mod to trust the user to set up and direct the chests to the entities they wanted filled, instead of doing it automatically, I could reduce the inventory checks significantly which would greatly reduce the UPS usage. It would just mean a little more set up for the user, and they would have to ensure their chest stay full rather than rely on The mod to automatically fall back to other ammo.
I wonder if this change would be well received though, since it means more management from the user? I may try and implement this and have it as an optional toggle for a while to see how it is received.
Thank you for your suggestion! It's giving me a lot to think about!
Let me know if you think of anything else, and I hope I can rely on you to play test this sort of implementation if I manage to make it happen!