An "undercar" perspective that hasn't been displayed yet... your 91-01 diffuser will be roughly 3 inches shorter in the rear, just about equal to the exhaust tip projection shown below... I am not sure how much this will affect the relative performance of the diffuser.
The feel at speed is noticeable. Car velocity will only change upon concerted driver input... the sensors in your body will detect considerably less unexpected behavior as the car translates. Chaotic air pressure gradients will be greatly diminished. You can almost envision riding on rails of air... if you want a boat to go straight in the water, add lots of long fins to the bottom... and then try to turn it! Thankfully, air viscosity reduces the required force to alter vector...
I have been strongly considering affixing some streamers and having another car(s) with a goPro/equivalent recording some runs "at speed" and stills as the car passes by... it may not be windtunnel testing, but like rain, it will give a visualization of the air stream / pressure gradient. With a little more involvement, I bet you could even start to estimate the additional downforce... anyone have a daq board and some accelerometers sitting around?
A good friend of mine ran sprint cars for hundreds of races as a teen... he created his own airfoil design without a wind tunnel, a design so effective that it was banned after 1 season. That is to say, high cost testing is efficient for improving the final bits of possible performance, but it is often possible to iterate and progress a design with simpler, focused testing and sound design principles.
One phenomenon that one might encounter upon detailed testing regimes is the relationships between the components of the car underbody airflow system. It is my understanding that improving other parts of the car's underbody aerodynamics will improve diffuser performance, and perhaps to a degree much greater than tweaking the diffuser itself. While Difflow will continue to improve their design, you may experience more in-car, seat-feel high speed stability as other aero is upgraded. The Difflow Diffuser represents the key final element in the airflow system... and the one the world sees as you shortly disappear before their eyes.
