First, I agree you will lose performance. The performance you will lose is a function of speed and load. Generally speaking, the amount of spark retarded due to a lower grade fuel ( as low as 87 octane ) is very small at part throttle, and increases more significantly as the load increases. Even after the system has "learned down" form the 87 octane, mid-range and normal cruise speeds probably are not effected form reduced spark. However, any time you would "put your boot" into the throttle, with 87 octane you will lose performance. Wide open throttle is already a very low MPG load point so over-all fuel economy, would most likely not be effected, ( or would be effected so little as to be unnoticable) if you drive at a reasonable throttle. But all that being said, even 20 cents a gallon difference in price is only $3.60 a tank !!!!!
And you never know when some PESKY SRT4 might be lurking !!!!