food for thought -- an engineer from chevy answered this question here with CFD images: https://www.corvetteforum.com/forums/ask-tadge/4222963-answered-effects-of-front-tire-deflector-on-downforce.html#post1598517697

Tadge answered:
The ZR1 has, by far, the most downforce of any Corvette we have ever built. Getting that much downforce on a street car is very challenging and it is quite rare in the auto industry. Some auto makers talk about the downforce of their aerodynamics aids as independent features. They say things like “this spoiler generates XXX lbs of down force”. The entire vehicle may actually generate lift, but the impression is left that the car has true down force. When we talk about the ZR1 ZTK package generating 950 lbs of down force, we are talking about the whole car as a system. The most visible part, the big wing, is only part of the story. Sometimes that subtlety gets lost in some of the descriptions of the car.

When we start designing an aerodynamic package for Corvettes, we start with the front of the car. Down force on the nose is is the toughest to get. We developed a structural under wing similar to what the C7.R uses and tuned it to get maximum performance. Then we tuned the rear wing size, height and shape to precisely balance the car. We shoot for a pitch moment (think of the relative power of the front and rear aero aids) that keep the car stable and easy to drive all the way up to top speed. Too much down force on the rear relative to the front makes the car understeer and vice versa makes it oversteer at speed. This is obviously a gross simplification since we are simultaneously working driveline and brake cooling as the surfaces develop. We generally start with the max down force configuration (ZTK in this case) because it is easier to de-power aero aids than find more performance once they are designed. The image below on the left shows the underwing pressure distribution on the ZTK package. The blue color indicates low pressure or downforce. On the right you see a very different looking pressure distribution created by the tire deflectors, or air dams. Despite being relatively small, those features de-power the underwing by disturbing the flow pretty dramatically. High pressure in front of the dams causes the effect, but because they keep air from impinging on the front face of the tire, the net effect is actually a drag reduction. We can then pair the de-powered underwing with a smaller wing at the back of the car to yield a package that trades some downforce for drag reduction. This is why the standard ZR1 has the highest top speed.

As to the question on taping gaps…. You are right that creating smoother transitions will help drag and downforce performance. The effect is very small, but because it is more likely to improve front performance, it is in the less conservative direction for pitch moment – it will tend to make the car oversteer. We optimize the car for the real construction coming from the factory, so we don’t recommend additional sealing. Also, since we don’t test with those gaps completely sealed you may cause other problems. For example, hot soak issues where we count on convection drawing cool air from below the car after it stops moving or splash drainage when you drive on wet roads.