Also in my post where I brought back your pics, I mentioned Tuft Testing. This is real world aero testing, and if you take pics or video during can shut quite a few people up. Again like I said, you will ultimately spend the rest of your days debating the whole drag vs downforce thing....
As to diffusers, my front diffuser made a huge amount of downforce. My rear diffuser did not, but what it did do was move the rear axle turbulance that buffets the back chassey around behind the car allowing it to cut through smoother with less lift. make sense?
I can't help you with if your car is slow or not. For me ultimate speed is not the goal, so much as precision and control are. Don't worry about impressing interweb people with how fast your car is....