Any of the additives out there are surface tension reducing agents. They make the water more slippery so it flows faster thru the system. They won't in most cases reduce temps. Water will still boil at 212 degF and with a 15 lb cap it will boost boiling to 257 degf. For every pound pressure cap you run it boosts the boiling point of water by 3 deg per lb so a 15 lb cap boost boiling by 45 deg.