I REALLY dont think you want to be pumping water into your headers under 1500rpms. Thats the reason for the valve. 
Marc's boat has through transom headers. Marc's boat's headers have 5 water inlets per
each header. 4 of those water inlets are on the header where it attaches to the cylinder head, just like "normal" boat headers. The 1 other water inlet on those headers is in the
collector, which is
after the header rises and falls below the boats water line. So although I agree with you that you don't want water in the 4 water inlets below 1500 RPM's, I disagree with you that you don't want water in the collector below 1500 RPM's.
Let's see what Marc's reply is as to how many RPM's he was turning when the header collector's rubber hoses failed. Marc?