Mileage and emissions are the only meaningful innovations left for the internal combustion engine.
For over 100 years, people have measured horsepower and torque. These measures ceased being interesting decades ago. Add fuel and air and the engine produces more horsepower and torque. Always has. Anybody can do this, even the former "big 3" US makers and Subaru.
The state of the art is therefore not producing more horsepower and torque, but producing adequate horsepower and torque while using as little fuel as possible and emitting as little as possible.
This definition of performance seems to be supported by its implied "future-proof" qualities. By that, I mean that if we take my definition of performance to its logical extreme, the best internal combustion engine is... no internal combustion engine. I believe we are heading that way, as fossil fuels are not sustainable energy sources (they are finite, and "dirty").
Fuel efficiency is more important to me in a daily driver. My Forester has plenty of acceleration to safely merge with traffic, so does my wife's Civic..in fact, as a people mover, I think both are a little overkill.
As for my "toy" car (WRX), I'm not really concerned with MPG. In a car that's solely used for fun and not needed to use for transportation, anything over about 6mpg is acceptable to me.