I'm not a Subaru guy, but I know enough to know that the cooling system is a weak spot, and I've read enough on this site to know that nobody here (aside from maybe Carey himself) knows more about Subaru cooling than Jim Kelly. But as a guy who deals with heat transfer for money every day, I can tell you his long post above is true in every detail.
Water is a more efficient coolant than glycol (polyethylene, polypropylene, etc.) . There are always other considerations, but for straight heat transfer, glycol of any kind reduces efficiency. The stronger the mixture, the weaker the heat transfer.