Side plates
Plates increase the downforce somewhat while increasing drag and also provide sponsorship surface. As a vehicle starts to slide sideways, endplates will disturb the flow on the outside of the airfoil or spoiler and increase downforce on the inside. You'll lose some forward bite due to downforce reduction in the slide. Sprint Car endplates are ridiculous as an aid to the wing. They act more like a rudder or a barn door.

They cause the inside lean, or reverse roll angle and they're not allowing the car to perform its best way. The wings on Sprint Cars are set at angles way past stall. The added drag rotates around the contact point of the rear tire and the ground, causing a moment that transfers load from the front wheels to the rear wheels trying to lift the front wheels.

The front wing is also stalled, but in addition to the stall, the close proximity of the hood interferes with the predominant surface doing the work. A piece of plywood would work just as well as the wings on a Sprint Car, partly because of their regulated airfoil parameters and partly due to the stall angle. Endplates help more on formula cars. A properly designed wing sees zero effect from an endplate.

NACA Ducts for bringing air into the driver's compartment
These need to be placed somewhere where the yarn tufts are attached and straight. If they aren't, you may be placing these in a suction area and will get little flow, if any at all.

Exterior Mirrors
These should have a short vertical height and be as wide as possible for minimal drag. Their actual shape will change overall drag, and depending on how they are attached, can make lots of drag.

Brake Ducting
Scat tubing used for brake cooling and other things has its benefits. The benefits are bendability, and flexibility. At lower velocities they work pretty well. At higher velocities though, the effective diameter is much smaller than the measured diameter, significantly reducing airflow. You should use minimal scat tubing to transport air. Make ducts that have smooth insides, with blended curves. Keep the air velocity low.

The more smoothly surfaces are blended, the better chance you'll have for drag reduction and better downforce. Don't think that steep surfaces make more downforce, because they don't all the time. Most cars today have gone way beyond the best angle, and more is not always better. On the other hand, significant drag reduction and downforce improvement will require testing and optimum chassis set ups. Gains may require a slightly different driving style.

On dirt cars, there may not be enough braking force and traction to slow enough for the corner when you get the aero right. In fact, this may be the unintentional reason that the aerodynamics are stalled. Right now, many types of race cars are wasting lots of horsepower overcoming high wasteful drag. A Sprint Car on a quarter-mile, relatively slow track, probably won't see a gain here, but on longer tracks and with all other cars, there'll be a gain if you pay attention to what you're doing.

Every type of race car can achieve gains in lap times from paying close attention to the above issues. These includes Formula 1, Cup, Late Models, Sprint Cars, IndyCars, Grand Am Daytona Prototypes, and Sports Cars. Don't assume that because another team has the funds and resources to test that it has it right. The other team may just have it more right than you do.