If I understand correctly, the heat grade of a spark plug is a trade-off between keeping the plug hot to burn off deposits, and keeping it cool to avoid pre-detonation. Car manufacturers will bias this trade-off towards hot because it keeps the service interval longer. Whereas - we ecomodders - could bias this cooler because we don't want pre-detonation. Once the ECU detects pre-detonation (knock) the timing will be permanently retarded until the ECU is reset. Retarded timing means less power is extracted from the fuel, and so worse MPG.
The NGK website says
Spark Plug Terms
"When making spark plug Heat Range changes, it is better to err on the side of too cold a plug. Running too cold a plug can only cause it to foul out, whereas running too hot a plug can cause severe engine damage."
So worst case, cooler plugs would just mean checking the plugs more often for fouling.
Will this work to improve MPG???