[hayden55]

I would say you are just adding to excess flow and it probably requires the throttle body to close more to maintain 55 mph now without speeding up. There's a reason why the prius uses EGR to control RPMs in the mid range on the prius. Allows the car to open the throttle body just about wide open the entire time and pump the intake manifold full of an enert gas to essentially "throttle" the engine as if it had a small wide open throttle body. Doing smooth transition 180 degree itty bitty headers would work the best i assume but people typically aren't going to go to all of that work. They will just do a cheap short tube header with the oem cat and move on if they care about low end efficiency.
So go small to increase torque down low for mpg or go large to increase up top for horsepower (torque x rpm/5252 = hp). They are inverses. The try y's and smooth bends help efficiency.
This reminds me of the guy with the lawn mower carb on the v8.
Also, this is the reason why people try and pulse and glide at the highest percent load they can get in closed loop without going to open loop.