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Actually getting up to speed is what kills the mileage in the subaru. All rotating mass will greatly effect TP and engine load required to get up to speed. The "back pressure" is a mythical creature, you can run a car on open headers. Back pressure could become an issue with there were cam changes, head porting, time changes, etc that would effect the duration in which the valve is open to exhaust. But there is nothing that requires such a change from a small difference. Plus, the exhaust already has the factory muffler replaced with a two chamber, adding a bit of back pressure at this point won't account for as much that was eliminated to begin with.
As for the "load" placed on the alternator, most modern units are equipped with regulators. The "dim" of a headlights is actually the rate which your alternator responds to adjust to compensate for the load. If you idle is changing when the alternator dims you most likely experiencing a soon to be failed alternator, poor grounding of the electrical system, or some other variation in the ignition system of the car which was greatly effected by a change in electrical demand (aged battery, old coil, etc).
Of course issues might arise of this change, but the simple fact still remains no one has procured a unit which does or does not work. Or has/has not effected FE in a good/bad effect.
And guinea pig numero uno will not be the Subaru.
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