You are ignoring the bell shaped restriction on the inside that allows one to read flow rate? I have similar:
http://www.procooling.com/~phaestus/flowmeter.jpg
HUGE restriction regardless of the fittings used. Also mine is rated +/- 6% accuracy; is just using the pressure across the radiator any worse? Certainly a lot less counterproductive than lowering flowrates considerably to measure them....