It all depends on your needs for the car. If you have short commute it might not matter. If you put 50,000 miles on a car in a year it does.
National average miles driven per year is around 15000.
If gas is 3.00 per gallon and you drive 15000.
500 gallons in a 30mpg car - $1500 on gas per year
600 gallons in a 25mpg car - $1800 on gas per year
750 gallons in a 20mpg car - $2250 on gas per year
According to fueleconomy.gov
06 GTO M6 gets 23 hwy / 15 city
06 RX8 M6 gets 22 hwy/ 16 city
06 Vette M6 gets 26 hwy/ 16 city
02 M5 gets 19 hwy/ 12 city (for comparisons sake)
I undestand that there are other variables and observed MPG but just giving you third party facts. It seems the difference in fuel economy and cost per year is not as large as one might think.
Power difference on the other hand is obvious.
My buddy with his 06 M6 GTO says he can get 25mpg on the highway at 70 - 75 mph if he is careful. Drop it down to 60 and he can see 28mpg. I am sure a vette can do even better. But not regularly at highway speeds.
My wifes 00 Impreza Wagon is lucky to see 25mpg at 75 on the highway. GRMs WRX wagon got about 22mpg on long trips at 75-80mph.
Who really goes 55 mph any more? Hell my modified 1st gen RX-7 has seen 27mpg at 50 - 55mph. It drops to about 22mpg cruising at 80mph on the highway.
I love GTOs, Vettes, CTS-Vs and the like. I also love the RX-8 but it annoys me that people think that anything with an LSX gets 30mpg.