I know what you're saying,
and it is true that in most cases aftermarket exhausts are designed for the "Rev Head" who wants to push more through the engine in the pursuit of power, but that is not necesarily the whole story.
It is a fact that OEM exhausts are a compromise in just about every way, economy, performance, noise, space, etc. in no way do they represent state of the art for economy tuning.
An exhaust can be tuned for free flow to get economy, it's not all about big pipes, it's aerodynamics and optimizing flow for a particular set of conditions. Smaller pipes that are properly configured, length and shape can be used to improve engine efficiency but you'd be hard pressed to find an exhaust shop that actually knew what you were talking about.
The article below talks about some of the intricacies of exhaust design, there is very little out there re economy, but maybe that will change over time.
Exhaust System Technology: Science and Implementation of High Performance Exhaust Systems