At the risk of this coming off the wrong way, why did Ford not equip the car with 103s at those specs from the factory if it got better MPGs and more power? I'm not stating it to be argumentative, but I figure that if it meant bigger numbers for power and MPG on the window sticker they would have done it, not like it is a cost/comfort issue like a cold air box or performance exhaust system.
Life is the only thing I can think of, but based on the sparkplug thread here people get 30,000 (at the factory gap of .050), which is about average for copper plugs in the American engines I've dealt with, and it isn't that they are a pain to change. Platinum would seem to be the ideal compromise then from a factory standpoint; good life and while not as good performance wise as the plain copper, still better than the higher heat range. That's my hang up in switching mine over to 103s; if they worked better why weren't they put on from the factory? Trust me, I want them to be better... I am not fond of the way these cars idle and a little MPG boost always helps when you put 360 miles a week on your car.
By the way, what type of fuel are you guys using when getting these gains? Regular or 93, 100% gas or 10% ethanol crap? Any performance ad ons?