1.) How much worse will the fuel economy be? Is anyone smart enough to do a back of the napkin calculation on the increased RPM and energy losses to friction? If it's significant enough, it could be a factor...especially as fuel prices will certainly go up over the period of time that many of us will own these cars as daily drivers.
Negilgible. The gear ratios are still so wide you won't notice any difference at all when driving, and RPMs are only 285 RPMs higher at 70 mph compared to running the stock 3.55 ratio. See below the cruising RPMs and max speeds at 7400 RPM in each gear.
2.) How would the ratio impact the longevity of the motor? Would you shorten our oil change interval? Does it notably increase the amount of heat that the cooling system has to deal with? Does this mean that non-PP owners need to budget for the PP radiator?
Negligible if not at all. Your oil life gauge will tell you when to change the oil if you like to use that. No extra heat, it's all fine.
3.) Would Ford void your warranty for this?
4.) Most importantly, Is it worth it?? What is the new ratio like to drive? Does it measurably improve performance?
Obviously. Without it your 2018+ GT is slower than a 15-17 GT with less horsepower up to about 80 mph due to their better gearing.
Watched this yesterday, I really wish they hadn't made this change