Two heat units colder sounds like a lot. Too much. I would expect the plugs to get carbon fouled, unless you do a lot of high speed driving for extended times. I do quite a bit of slow-ish driving around town and every time I've asked a shop if I should go to a higher heat range to burn off carbon they always say no. They say the mfgr suggested plugs are best for a wide range of driving conditions.
Also, the wider gap can be helping you now, but as your ignition system ages you can end up with a lot of misfiring if your system is no longer able to supply the high voltage needed for the wider gap.
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