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One of my cars had last silencer such that pipe entered and exited from same end, replacing that with freeflow type did improve throttle response and also 0.5l/100km decrease in fuel consumption, which I could see happening right after exhaust replacement and it was permanent drop.
However I did not do any other testing than just noting consumption of each tank, would be interesting to see some proper testing with different kind of vehicles and different type of driving.
One thing that might easily affect to results here is sound, driver can sub-constantly use less throttle when car is louder, which can be also affecting freeflow air filter testing. Less fuel saving mind might actually push pedal deeper because of same reason, so testing should be done with cruise control, I think it is more crucial with these mods than with others.
NA motor exhaust pulses are important when trying to make most power at least, those are used to flush out all exhaust gas in performance motors, for that reason measurements are rather important.
It should be possible to calculate exhaust to have best gas removal for certain rpm, as it is possible to calculate intake runners so that at certain rpm there is kind of 'boost' from recoil pulses which fills cylinders much better, exhaust must then match to intake and both need to match to cam etc. That way it is possible to get quite few % more power from engine, perhaps also better economy, but changing only one of the components is not giving much improvement if there was nothing wrong with tune up in first place.
All those + modifying cylinder head should give gains to at least older cars, where swirl is not best possible and intake manifold runners are not so good. Cylinder head modifications especially are beneficial at part throttle situations, but to get best it have to be looked as whole package.
That is at least what tiny understanding I have been given from subject.
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