I doubt the Mustang is that much faster and the better MPG is only because of better gear. The Mustang has long gears since the engine makes more torque. The M3 on the highway revs at 3000rpm at 120, I bet the Mustang revs at 1500-2000RPM at the same speed.
I average 11.5/100L in the M3 but short shift from 1st to 4th in the city, However once you open the ITBs expect 17.5-21l/100. The Mustang will be the same deal. Those numbers quoted are just numbers for docile driving. I don't see the Mustang getting more then 500kms a tank, which is about the M3 gets under normal driving. Real world is much different compared to what the EPA says.
As I said the M3 is only worth it in 4dr trim. If you want a coupe then get a Mustang but if you want something that does it all and you can fit people in the rear set and golf clubs in the trunk the E92 M3 is perfect.