I think it is generally accepted that Ram Air is good for about 1" of manifold pressure.  When you're at altitude, that's a good boost.  Will it give you 10 knots of speed, I doubt it.  Of course operating on Ram Air in icing conditions would be a very bad idea.  I'm sure you could fabricate a 'filtered pseudo ram air' without too much loss of performance, and would require no pilot action.

      D. Story