The link shows results of testing Lotus Exige in the wind tunnel with all kinds of spoilers, diffusers and flaps, as well as in stock form. You can see that the test was performed at 100 and 150 mph, and the calculated coefficients of drag and downforce on the front and rear axles do not differ much (they are within the error range of the measuring equipment).
This pattern can be traced in the declared characteristics of automakers. For example, for the Ferrari Enzo, it was claimed that due to changing positions of the movable ribs installed in the front ventilation ducts and diffusers, at 200 kph, the downforce increases from 190 kg to 344, and reaches 775 kg at 300 kph. At 350 kph in low drag mode, the downforce specified is 585 kg. If we divide these numbers by the square of the speed (in m/s), we get the following picture:
This means, that for a car in "high downforce" or "low drag" mode, the downforce coefficient doesn’t change at different speeds.
As for the lack of a data, for the last decade, for almost every track oriented car, manufacturers had given some information about downforce. Sometimes these are specific values, sometimes something like "15% higher than its predecessor". In any case, I think it would be interesting to systematize and compare all this information. Since we already have "emissions" and "fuel consumption" in the car pages on this website, why not add downforce?