Seeing how there's been no definitive answer from someone in the know, and all the speculation done by everyone, we really don't know which port is truly the fill level hole.
I have been running mine filled to the "vent" since I did the first service on it and have had no problems and it runs more quietly.
In all the years I've been working on cars and equipment I have yet to see a gear box that isn't filled up to the sight port. VW, Porsche transaxles and pretty much every straight axle rear end as a couple of examples.
As far as loosing horsepower, 4-6 oz of additional fluid is not going to create enough drag that you would notice.
Windage trays in race engine keep the high speed rotating mass of the engine out of the splashing oil in the oil pan. Those parts have their own pressurized oiling passages and do not rely on an oil bath for lubrication.
Where as the gears in a rear end or gear box spin much slower and rely on being dipped in oil for lubrication. So that isn't a very good comparison to make a point about drag.
And contrary to opinions expressed you aren't going to blow the seals out or ruin bearings by having "too much" oil.. There's a vent in the top port for a reason, to equalize pressure as the oil expands and contracts due to temp changes.
