Believe the out and back speed difference isn't the same guys. In other words for the same wind speed, the amount of increase in drag is not equal to the decrease in drag with downwind.
This I believe is due to how the drag coef. is affected. Into the wind, even leaned over body has a higher drag coef. compared to reversing the bike in a wind tunnel with the same wind speed. This means the body has a higher drag coef. into the wind than downwind. Its further complicated because a rider is always riding into the wind on some level.
So I believe the math is more complex than described because the drag coef. isn't the same for the same rider into the wind compared to downwind. Just some further thoughts. Thanks for all the great comments and math back up.