As far as your belly goes, you have a really good start, but you could definitely improve upon it, especially if you moved the exhaust pipes. If there is nothing hot running down the underside anymore, you can do a near complete belly pan to fill all the "holes".
Here is a rough idea of the template on your car. You'll need to fill in the voids as best as possible to lower your cD.
Is there an option for a spoiler from the factory or an aftermarket one that is specific for this car? If you don't want to do a kammback, a spoiler might help fill the void and still look OEM.
A rear diffuser would also probably help considering the sharp, vertical cutoff of the rear bumper. A clean belly transferring to a properly design diffuser would help a lot.
Do you have any pics of the new exhaust setup?