I guess I'll be eating my hat then. I'm pretty amazed that you'll be able to keep it planted that fast around corners, but if you can, this will be an incredibly fun (or maybe very boring!) car to drive.
And the Roadster doesn't have real side skirts to seal up the tunnel, which significantly degrades any diffuser downforce.
Just pretend, the same way they do with the claims about the GTR's ~0-degree wing, and most all aero claims on street-legal cars.
Aero at street legal speeds is basically worthless and range-destroying. Many supercars don't have more aero than the Roadster, just go look at them, they usually either have no wing, like the Lambo Huracan, or a wing with virtually no angle of attack.
As a matter of fact, F1 cars in the 70s-80s were using venturi tunnels that extended the entire length of the vehicle. This is impractical in a modern gasoline-powered car.
Theu were even getting so good with their aerodynamics that they were rumoured to be generating more downforce with ground effects than from wings.
In an electric car, you could do what they were doing back then.