No significant difference. There was a 4 hr difference between the 2 runs due to letting the engine cool so I can change the plugs and driving 30 minutes to let the ECU settle in. The weather did not change during that time. The procedure for changing to the halos in the instructions is to reset the ECU so it can adjust faster to the faster firing sequence. I think that if you havent tuned up the car in a while their procedure of resetting the ECU along with the new plugs might give the reported gains of 10%. http://www.haloplug.com/
They do have data that shows an increase but not for me. Your mileage may vary.