Super- template spoilers
 

The following is a sampling of vehicles which embody aft-body trailing surfaces, either as the actual body, or created with the addition of a spoiler, which exceed the profile of a streamlined profile they originate from, presumably to create 'direct' downforce :
* FORD GT 40
* FORD Mustang MACH 1
* Ferrari GTO
* Buick Wildcat concept, 1985
* Chevrolet EXPRESS concept, 1987
* Lamborghini Countach 5000
* Pontiac FIERO
* Ferrari F40
* Saleen Mustang S-351
* FORD Mustang GT, 1994
* Ferrari F355 Berlinetta
* Mitsubishi Eclipse GSX, 1995
* VECTOR M12, 1996
* McLAREN F1 GTR LeMans, 1997
* Mercedes-Benz CLK-GTR, 1997
* Lamborghini SESTO ELEMENTO, 2010
* Porsche 918 Spyder, 2010
* Bugatti Veyron 16.4 Super Sport, 2011
* Lamborghini Aventador LP-700-4, 2011
* Volkswagen NARDO concept
* Porsche 911 GT3 RS 4.0, 2011
* Porsche 911 GTR RS 4.0
* Nissan GT-R Black Edition, 2012
* McLAREN MP4-12C
* Chevrolet Camaro ZL1, 2012
* Mercedes-Benz C63 AMG Black Series Coupe
* SHELBY GT 500, 2012
* Audi A7, 2010
* Nissan LEAF NISMO EV Racer, 2012
* Aston Martin Vanquish, 2013
* SHELBY SUPER SNAKE, 2015
* Ferrari LAFERRARI, 2015
* Bentley Continental GT3-R, 2015
* Chevrolet Corvette, 2015
* SCION FR-S, 2015
* Lamborghini HURACAN LP 610-4, 2015
* Porsche CAYMAN GT4, 2016
* Cadillac ATS-V, 2016
* Chevrolet Camaro SS, 2016
* FORD MUSTANG SHELBY GT350R, 2016
* FORD GT, 2017
* SINGER 911
* Mercedes- AMG GT63 S 4-Door, 2019
* McLAREN Speedtail, 2020

What is the purpose of such a list. If one could 'average' the full set to a generic *template* you might have something.

purpose
 

1) These are 'direct downforce' spoilers.
2) They may originate on a car that had separation-induced lift, and correct for that as well.
3) They may have originated on a car with zero-separation-induced lift.
4) They all terminate outside the TBL, and are impacted directly by the local streamline.
5) The 'template' merely serves as a reference datum.

And how did you determine all of that?

Technical papers on each car?

Coverage of the cars in aero textbooks?

Talking to the designers?

Personal testing?

Or is it all just guesswork based on your theories?

But still, what's the utility. Maybe they could be sorted into groups?

Use case drives everything (I count 45), aero is along for the ride.

utility
 

If you look at the top speed of all the cars you realize that it's done for safety.
They have no bearing on fuel economy improvement. 'Bacon-savers', although with added drag.

That may not be true. Porsche, for example, reduce drag with their rear spoiler. My Insight has measured reduced drag with its custom rear spoiler.

Without seeing the test data, it's impossible to say whether the spoilers on these cars increase or reduce drag.

Rules of thumb and guesswork in car aero is not a good idea.

Porsche
 

Which one?

Which rule of thumb or guesswork? All of them.

As for rear spoilers being able to decrease drag, consult any recent aero textbook.

how
 

'The object of analysing aerodynamic drag is to determine the relationship between cause and effect. If each detail's contribution to drag could be defined and then minimized, a vehicle with the minimum aerodynamic drag would be obtained...'
If profile drag dominates aerodynamic drag, there is value in analyzing profiles which minimize it.