Backpressure is not what boosts low end performance, rather its the effect of having higher exhaust velocity at lower voulme. Certainly introducing deliberate restriction into exhaust by introducing some kind of contstriction further down the exhaust will cause loss of efficiency at all speeds.
I think some exhaust mods do gain MPG, most exhausts are set up from the factory to resonate and muffle effectively at cruising speed, this assuredly does sacrifice some efficiency. This is partly why swapping mufflers around and removing the resonator can lead to a droning 'fart can' noise - even if you haven't picked noisy performance mufflers, the factory exhaust had that sound tuned out.
My car (same as a US domestic A32 body Nissan Maxima) has a restrictive U-bend in the y-pipe between both banks of cylinders and the inlet to the cat, this is designed to keep the car quiet and muffle the V6 booming sound accross wide range of speed. It keeps the exhaust cabin noise quiet even with a rather feeble rear silencer. Replacing this alone with a better collector on this car is known to be single-mod gain of 10-15hp and sometimes more (not necessarily mandrel bent nor equal length even!). I'm planning to do this mod, as it is so close to the engine and is bound to be reducing performance at all speeds. Again it's a resonance thing, not a exhaust flow thing. If it's silencing the car at a given speed then I suspect there may be gains to be had.
Not expecting much though, maybe 1-2% in typical driving? I'd see better if towing or climbing hills though - and indeed my daily 20km round trip commute includes 500ft of evelvation change mostly being in one steep fast run.
(Do they make multi-lane highways climb such steep hills at open road speed limit in other countries or are our highway engineers just a special kind of mad?)
