Acorn,
The OGE power plus 5% is a very high margin indeed. The difference between OGE and IGE is about 15% and if you add 5% on top of that you lose about half your payload. That is a very conservative rule.
No performance is lost because of the downwind at all. The ground effect does not change because of movement of the air or the ground, that is an ancient piece of pilot lore. Think of the big bombers, with 1 of 4 engines running, limping back across the channel, 1/2 wing span above the waves. They had no trouble gaining ground effect!
For all the ppruners who would like to try, I invite you to make a careful downwind landing and note the power (use manifold pressure or engine turbine temp) needed to complete a careful approach. Compare downwind to that needed for still air, there is no difference. Ground effect is not lost because the aircraft is translating, that effect is nil.
Regarding the safety of downwind takeoff or landing, it is clearly less safe, due to barrier clearance, failure tolerance and ground covered (roofus shows great wisdom, why not back up across the clear area to the rear?)
On the other hand, I have had to do hundreds of downwind landings, and dozens of downwind takeoffs in my career, and knowing how is part of our craft.