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I would say a higher flowing exhaust will help unconditionally if you keep your foot out of it.
It has already been mentioned that reduced back pressure means a higher manifold vacuum to maintain the same power output. A higher manifold vacuum means that you will have a greater crank case vacuum via the PCV valve. This will reduce the amount of air being pushed around in the crank case and reduce drag on the engine. A higher manifold vacuum also means better fuel vaporization (but this results in negligible improvements to the percentage of it burned).
As to whether it will give a 3 to 5% mileage gain, I doubt it. When cruising at speed flow rates through the engine are low, and thus the back pressure is low. Improving the flow of the exhaust will lower back pressure a tad, but squat minus squat is still squat.
It should help but I doubt you will be able to measure it with any certainty.
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