..and I think therein lies the issue - other people will always make assumptions about your car, what you have done to it and why, which on the whole are not usually correct. If you are bothered by them then dont do it. If however you are doing it simply because YOU like the look of it then by all means go for it - looking at the bumper in isolation, it looks fine.
You could argue that everyone on here who has a wing fitted to the back of their car is in a similar boat. Average speed in the London area for instance is about 10mph, why have a wing on the back if you never need the downforce 99% of the time? I mean, you are not driving a 500bhp racing Porsche so why are you putting on parts that you find on a 500bhp Porsche. But, personally I like the wing, it looks fine.
I'm not from London and my car gets driven hard. The wing I'm putting on soon will be functionally aswell as authentically pleasing.
What your not grasping is the fact your making a car look like another. Make it unique by all means but don't imitate.