At the speeds you're talking about, it won't do anything.
If you like the look, then by all means go for it. Just be prepared to wake up one morning and look out the window and realize you've ruined your car. Be ready to buy a factory replacement trunk lid in a few years, because unless you turn your car into forced induction track monster, you will invariably get tired of the look, or at least get tired of other motorists pointing and laughing. Good luck.
