How did you end up attaching the final kammback? Just some tape on the top?
Have you considered doing a second windshield wiper cover that actually covers the wipers (kind of like doing a wheel skirt and full moon wheels together)? I'm thinking any air that gets in there might find its way over the windshield a little easier with a second cover.
What about a full grill block that opens when you need it. You can do an electronic one like some here have done or a passive one that only opens when the air isn't hitting it (like stopped at a light). A full block will give you just a little bit more of a cD drop.
You could get some interior mirrors and delete the exterior ones. Try doing a search for a fouring mirror. Or this:
Fouring(Made in Korea) BL Blind Spot Mirror Wide Angle Rear View For Right Driving Position : Amazon.com : Automotive
If you can get rid of any accessories attached to the engine by a belt that would also help (a/c, power steering, radiator fans, etc.) You can also replace some with an electronic version, which might use less power overall than the belt type, especially if you charge the car overnight using an outside electricity source.
You mentioned lowering your acceleration RPMs. You can install a turbo which (if you keep your foot out of it) will help you get decent acceleration performance with low RPMs.
Also, since we have the same car, make sure it's not burning oil, these model engines are notorious for the oil return holes on the piston oil ring to clog causing major problems and the oil burning. Is your tailpipe tip black?
What about the little blocks in front of the wheels to keep the air going around the tires. (I can't remember the name of them right now).
Maybe an undertray and rear diffuser?
I don't know how you would make it to access the trunk, but you can do a full/partial boat tail off of the bumper to go with the kammback.
You can extend some cardboard on the end where the side of the car meets the back side. Extend it out like a kammback and angle them in just a little. This will help the air angle in like a boat tail without limiting trunk use.