The California Special doesn't have a hood scoop. That package comes with a side scoop and a spoiler.
If you're looking at adding a hood scoop to your GT I'd suggest looking at a 2010 with that feature. The normal hood is raised in the middle and the hood scoop raises that part 3 more inches. Short people will find it distracting but some people really like the look. It does look aggressive.
Aerodynamically, the hood scoop would have a very minor negative effect. I've never seen anything published that showed the difference.
2011 Mustang GT/CS - Race Red, 5.0, 6-speed manual, 3.73, Comfort Package, Electronics Package, HID, Tinted Windows, Boss Splitter