Enya engine - an assymetrical wing doesn't hurt the axial stability as much as the performance of inverted flight. An assymetrical wing will create lift without any angle of attack, which is to say it is parallel to the line of flight. Thus, to hold level flight inverted, where you must fight the effects of the wings normal lift (which is now pulling the plane down) you would need a more reverse elevator to keep it level. On a symmetrical airfoil, lift is created by keeping the wings at a shallow but positive angle of attack. Thus level flight can be achieved either through trimming the elevator or balancing the plane so that it naturally flies level without trim. The latter method allows you to fly level inverted without additional elevator input.

Of course, all this is based on my knowledge of engineering and I'm sure there are far more qualified people than I to speak on this matters. But since your post went unanswered, this is my two cents.