Most machines seem to cap productivity at 300%, not sure if this is a hard limit or something that can be bypassed.
Edit: There are mods to bypass this limit, though an option in the mod settings would be nice to unify into just one mod, if you have the chance, it would be much appreciated.
That's a base game limitation. Having tried mods that bypass it, my experience is that it doesn't play well when you combine productivity, speed, and quality--you end up with a lot of end products getting dumped on the ground around machines when the output quality switches faster than the machines are able to empty themselves.
