Very interesting.
No reason why the 991 is any different in principle to an air cooled car as far as venting the engine is concerned.
So is that back to square one then or just accept that either route may have some marginal benefits?
Stumbled across this old thread but it doesn't really give any definitive answers
http://forums.pelicanparts.com/porsche-911-technical-forum/176135-removing-rear-reflector-cutting-airholes.html