I also use Koolstops, have for some years. But I fail to detect any difference in rim wear. I don't keep a record or anything, but the rims wear just the same. I am always very aware of the grinding sound coming from the front while braking after the wheels picked up some wet grit.

Maybe for dry braking, the rim wear is less. But the grit is doing the major wearing. Ever looked at brake pads like that? You can see the aluminium debris in them, Koolstops or not.