The best mods are the ones that increase "efficiency". With increased efficiency, you will get by definition increased MPG AND increased torque and HP.
If designed properly, a good exhaust system will increase efficiency.
However for a ECU or chip mod, how about a switch. Just flip the switch and go into the MPG mode or Power Mode.
Or that "switch" could actually be a throttle position monitor, that when it senses a rapid throttle input, it will automatically put it into the power mode because it assumes you are desiring more power.