If the car over-steers (nervous), even if you don't lose traction, a wing may help but only if this happens at speed. If it always happens, look at weight dist., tire pressure, etc. first.
If the car has neutral or slight under-steer at speed, the wing will slow you down and you gain nothing.
A wing converts drag to down-force, more angle = more of both. Too much angle: the wing stalls and stops helping (but steals huge power). Too little angle: may even create lift, because the angle is NOT to horizontal but to the natural path of air coming off the roof. This is generally slightly down, so even a wing horizontal to the ground gives some drag.
The best angle will vary with trap speed, you need more angle to get an effect in the 1/8 mile.
If the car feels stable, crank the wing up 2° and see if your speed is up.
If it still wanders or needs to be corrected, point the nose down 2°.
Not to be obvious, but: if you do a 180°, the wing becomes a lift force and unloads the rear tires.
Spill plates are for limited wing spans where more width is illegal or impractical.

Trivia: Mercedes had a large pop-up wing with very high angle attached to the rear deck of a 1955 (?) 300SLR roadster, activated by the brake pedal. Step down, the wing grabs lots of air and slows the car down. Similar to the dive brake on aircraft.