Beeman - I checked that link out but I'm not sure it's all that applicable to a car like the SLC (or any street focused car). The radiator in an SLC is located a few inches from the opening of the car - not enough room to fit any kind of duct that would make sense. The principal being pushed in the linked article is really talking about the importance of ducting and requires a LOT of length in front of the radiator to minimize boundary layer formation and turbulence. I'm not familiar enough with the GTM layout so maybe these design guidelines works there - but for the SLC this isn't going to be an easy solution to incorporate. A 7-degree inlet angle is basically a straight tube when it's only 5 inches long.
I won't reference the hillclimb car since that intake duct feeds the engine and isn't quite as applicable for a radiator discussion. However, the next car uses one of the front inlets for driver cooling - so the duct they fabricated ran the length of the front end before being somehow directed at the driver. That's a lot of real estate for a duct!
Later in the article the author references a Bonneville F1 car that had a very small opening area - everyone thought it was going to overheat but it did just fine at speed. And that's the critical element right there - at speed. That design, and I think the 20% rule referenced, is likely more applicable for a race car at speed than a car that has to deal with everyday driving conditions.
For "slower" cars here's what he has to say:
"In most club level motorsport people building their own cars don’t duct the air all the way to the cores of the radiators and some don’t even seal the gaps around the radiator. If you have a car like this and it has cooling problems then first seal the radiator to the bodywork so virtually all the air has to pass through the core. If that doesn’t give you enough cooling then look at ducting the air using a gently increasing expander / diffuser."
I think the key there is "gently increasing expander" - again, packaging issues for the SLC make a gentle inlet duct very difficult if not impossible. There's some contour/shaping of the bottom of the splitter, but I'm not sure if that was just because a transition was needed for mounting or if it was designed specifically for aero purposes (I'm guessing the former). You basically go from opening to full radiator core area in the matter of a few inches (if retaining the factory radiator location).
As for exit area - the 1/4 area rule is the recommended minimum. I believe he also talks a little bit about ducting behind the radiator and he basically says unless you've got CFD, don't bother. Just dump air to wherever it's going to be least harmful (under or out the sides of the car I think). I have to think about this some more but again, I'm not sure how applicable this would be for a street car. Coincidentally my radiator discharge duct has a minimum exit area probably close to that 1/4-1/3 number but that was a packaging-driven constraint. I was more concerned about airflow management than anything else.
I'd say if you're planning to build your car for maximum cooling efficiency at top speed your link provides a recipe that's probably a good starting point if you don't have access to CFD and don't want to hack/test/hack/test/hack/test a solution together. I'm just not sure these are good guidelines for a street car that has to deal with a much wider range of operating conditions. Such long ducts would drive some real packaging issues if you want to keep the radiator up front. Maybe if you did as Rumbles did and split your radiator into two smaller cores and placed them in the side pods, then converted the side pods into functional air tunnels (instead of using the rear entry ducts), then you could get a higher efficiency cooling design. But given the amount of cost, effort, and experimentation, IMHO the more optimized solution would be to stick with the factory setup, make minor body modifications, and see how the car runs. If you need more cooling the next easiest solution is to incorporate higher flowing fans or a larger radiator core. It seems to be working for most of the SLCs out there that I know of.
So getting back to the main thrust of your post - I think some SLCs have cooling issues but it's likely if you reviewed those builds you'd see something fundamental was missing (shroud, sealing, good airflow management, etc). I don't believe the SLC has a cooling issue by design (for reasonable HP applications) - but I'm paranoid enough to do a few things that I *think* will make it better.
If you're not familiar with the SLC layout I have a few photos on my blog here at this link. You can get a sense for layout, scale, and packaging constraints in these photos.
https://superlitesite.wordpress.com/2018/03/04/28-full-of-hot-air/
Somewhere down in the middle of this post is a discussion of my hood modification and a before/after comparison:
https://superlitesite.wordpress.com/2018/02/25/27-the-first-cut-is-the-deepest/
And at the very bottom of this post you can see what my exit duct looks like with the body on:
https://superlitesite.wordpress.com/2018/05/04/xx-splitting-h-air-s/
Michael Fling's modifications can be seen starting with post 394 of his build log:
https://www.gt40s.com/threads/fling-slc-build-thread.39723/page-20#post-523328
Frankly - I'm amazed he was able to package that turn in such a tight space, but his fiberglass skills are way beyond what I've acquired to date.
Best regards,
Cam