I can't say I have seen a kammback on a flat-backed vehicle before. Usually people go for a full on boat tail, or a base plate.
If you follow the template, you probably will see some gains, as almost anything can improve what you have already, but I couldn't begin to estimate them.
I would assume that if you make the kammback, that the air going underneath the car is going to start doing weird stuff at the back. The air on the top will be lead down, but the air emerging from underneath may just continue rearward. I am going to guess this would create extra lift, and drag (not more than stock, mind you). So if you make the kammback, try experimenting with a rear diffuser.