I’m looking into getting an exhaust system, but I’m prioritizing the fuel efficiency over the negligible hp gain. So with that being said anyone have any data or input on how a modified exhaust has affected their mpg?
Depends on how much of the Exhaust you're replacing, If you you're just installing a Cat-back/Axle-Back Exhaust you really shouldn't see a major Dip in MPG loss. If you do then it's most likely due to a heavy right foot. I haven't seen any MPG loss on mine & I'm running a Corksport intake & Racing Beat Axle-Back exhaust. These car sip on Gas so if so you're that concerned about losing MPG leave it stock. Gotta Pay to Play
It seems to me that at the lowest rpm ranges (if memory serves... i.e. just above 2500 rpm) the CorkSport Axleback system makes slightly less torque than does the stock system. If this is the case and if you drive an axleback equipped car in a manner that is the same... (meaning calling for the same acceleration) as the stock exhaust equipped car - then it will use a bit more fuel - depending on whether you typically linger in those low rpm regimes... versus at higher rpm's (where it makes marginally more torque than the stock exh. sysyem car) and will get a 'wee bit better gas mileage.
In answer to your question re the catback system... the answer is no.
Here is the graphic. Pink is stock, BTW, axleback is green, and cat back is red.
BTW, the range where they are all identical is 2200 to 2500 rpm's... Then the differences become apparent.
Also, two other things:
Viewing Corksport's installation of the catback system (I was suprised to see that) the undercar cat converter appears to be removed with this setup. Maybe that, by definition, is what a catback system is. I thought it meant that you replace the system, from the outlet flange of the underfloor cat... and back (but leave the cat. in place). Me, I would not consider removal of the cat... I hate following behind cars where the cat has been removed. The pre-cat (which is integral with the header in our Mazda6's - 3rd Gen.) serves to do the "catalytic conversion" when the engine is cold... until such time as the underfloor cat "lights-off" and then it does the heavy lifting.
Second: I can see why, now, my stock '17 Sport (with 6MT) feels so weak until above 2500 rpm. It's 'cuz the torque is weak, weak, weak - 'till it starts going up aggressively at that point.