You got it all wrong. Exhausting more crap reduces your fuel efficiency.
I have replaced every car I've owned with a newer, more powerful one. Every single one has been more fuel efficient.
My current car is a heavily modified S60-R Volvo. Yes, it is heavier than my old Supra, and it has 460hps at the wheel (with the AWD fuse pulled) But it is also a Ultra Low Emission vehicle, and the first time I had it smog checked, the guy did it twice, because all but one of the categories on the California Smog check form were 0 (Zero point Zero)
The guy could not believe him eyes nor his machine. I have a bigger (than original) turbo, a dual intercooler, and a modified exhaust. After every single one of these modifications, the power AND the fuel efficiency went up.
So right now, I have a car that gets 31.1mpg on my daily commute, which is 12 miles highway and 5 miles streets.