I'm not so sure a Kamm would help much without it being meticulously designed and built, and even then you might only get 5-7%. Reason I say that is your roof to rear glass transition area radius is freakin huge, there are papers done on this that show a big rear radius improves Cd a lot.
The rear spoiler is almost certainly helping you some, and very unlikely to be hurting you. It isn't a lot, but If you like it, keep it.
Have you pumped up the pressure on your tires? That's 5% right there if you have them at 30psi currently. Also, blocking any kind of upper front grill area would help for making you more aero and it will also raise the intake temperature since this is generally where the air going to your intake comes from. Forcing the air through the radiator first heats it. (news flash, right?)
Plug that puppies nose!!
