Wow thats a tuff call, I am thinking your particular car "looks" like it may be better off without the spoiler? the only way that spoiler could help is if the trunk below the spoiler is too low from the roof line to provide ideal air flow. the added height may put the trunk into a aerodynamic sweet spot by adjusting the flow angle of air/wind off the roof(and trunk). Often a spoiler adds drag but a well placed and designed one can reduce drag.
Here is a link to where the discussion on the roof angle that I was recently reading -
http://ecomodder.com/forum/showthrea...why-12732.html post # 10