Brad smith has a cast that covers the ions and importance. The interview is a bit awkward as the guy he's talking with knows water control and isn't a brewer but Brad walks him through each of the important parts and what it means to the brew. As for efficiency, I've seen mash temps play the largest role. As little as 5 deg F°F can drastically change the gravity. Also tightly packed (thick) mash can lead to less fermentables than a thin mash
