You are wrong mate, it is a simple as that.
The theory you present above, which is not even that, as you take bits and pieces of theoretical aerodynamics , along with certain terms which you clearly do not fully understand, and you manipulate these, in an effort to present yourself right. Even if the theory of aerodynamics was as you presented it and it actually was applied to the Evo at the degree you are struggling to make it seem, in applied pragmatical action things would not be as such, as it is not even close to what you are trying to make people here believe.
I will repeat my initial statement, if you want an airfoil type that actually makes use of the air speed and velocity at higher speeds, replace the stock item with a well designed one. Also removing the rear evo spoiler, either that is from an fq360 or from an Evo260, it will make some difference, not worth the while, only in very high speeds. In simple words the bigger the size and the better the design of the spoiler, the more air ( more air as in higher speed ,volume and velocity) you take advantage,the more downforce the airfoil will create. Also buy rear spoiler that do incorporate an adjustable blade, as the attack angle plays an important role in the effectiveness of the airfoil.
Below is a video of my personal car , some years back when I had her on an fp black setup, now is cosmetically different and it has a different engine and turbo setup.The car was roughly in UK DD figures at 650-700 hp on pump gas, a lot more on race gas.
Now I run a custom 6868 billet turbo and it sits around 800-820hp, pump gas.
The pictures present the car with the spoiler, and in the small runs is also with one. But the highway high speed run, which was done in an other night, was done with another person at the front of the car ( added elevation of front axle pressure) and no spoiler at the back. It was removed to avoid additional cause of drag.
There were no issues of instability of any sort. The highway run does include a couple of open high speed cornering as the road route of it dictates.
I still run the car without a spoiler with the current setup on spirited night drives, and there are no issues at all.
All the runs were done on b-roads and highway, during night time where there was no traffic, as there is no race track of any sort where I live.
Marios