I do not know about the diffuser just hanging down like that. I am one to go along and watch what you do and in the end it comes out nice, but that is really distracting to the eye and just looks wrong for some reasons. I do like the long tail and curve you have there. What about a higher diffuser on the sides and lower in the middle integrated into the design more than the just hanging out like that? Maybe shorter or something. I am not sure right now, but there has to be a good solution there.
BTW- Without the roof scoop looks much better I think and I also planned on taking it of all along.