Clean the stock IC's that you're testing. The interior gets an oily film over time, that reduces heat transfer. The external surface collects dirt, fins get bent, passages get blocked with pebbles, same result on heat transfer. The question I ask myself is, how much difference would these guys have seen going to a brand new clean set of stock IC's.

Make sure you're recording the outside air temperature and humidity. Record the IAT's as well as speed and RPM.

Pick a route to do this test that allows you to repeat under the same conditions. Accelerating at the same rate and maintaining the same speed.

Try lower speed and high speed tests.

National Motorists Association