If I were force this flap open, so that it was open all the time, would I get a warning light or message? FWIW I have never seem anything like this in my life!

http://www.realoem.com/bmw/showparts.do?model=GN63&mospid=47785&btnr=18_0485&hg=18&fg=20

No warning light. You could disconnect the vacuum line inside the trunk and plug the solenoid. No performance gain and maybe a rattle from the flap. Better to leave it alone.