I feel like most contemporary spoilers are simply for styling."The Fast and The Furious"inoculated the public mind with the notion that the spoilers portrayed in the movie were the hot-ticket for any performance-minded street-racer.Those spoilers would begin to produce measurable downforce at 120-mph,below that velocity,they're virtually useless.Since they are intended for downforce,by default,they increase drag.
In the 1970s,when rear spoilers began to show up,they were a paliative for poorly designed cars.Modern day cars would have no need for a spoiler,as,simply raising the height of the trunklid accomplishes the same thing, while adding useful trunkspace.
If your curious,a rule of thumb would be to measure the angle of incline from the back edge of the roof,to the rear top edge of the trunklid,as measured from the horizon.If that angle is greater than about 12-13-degrees,you'll have flow separation and high drag.If the angle is 12-13-degrees,flow will be re-attached at the trunklid,with a captured vortex spinning within that defined space,a smaller wake,and lower drag.
For a notchback car,the patented Ford Bi-wing spoiler,as used for the Sierra concept car/ Merkur XR4Ti of the 1980s offers the best drag reduction for anything cooked up that I know about.
An angled aluminum plate as created by CAR and DRIVER for their Crisis-Fighter Pinto project for 1974,made a very productive rear spoiler,projecting up at no more than a 30-degree angle to a line which intersects an imaginary 12-13-degree line measured down and back from the roof.P.S.,the "tunnel" style,hollow type spoilers need to be "deep",projecting as close to the rear windshield as possible.
Hope it helps,Phil.