Is it possible that it sort of got a bad reputation because it caused the SU carbs to (presumably) lean out as car speeds got higher? That would seem to be a problem with the SU carby not the idea of cold air intakes....
I would have thought based on the location of the Mini carbs at the back of the engine right over the exhaust that a cold air intake would have more potential than on most cars.
I have a weber so I'm hoping the leaning out shouldn't be an issue, good air flow has to be being restricted by all the stuff in the engine bay not to mention the air intake sticking straight in to the firewall.

Agreed you might have more worthwhile uses for your engine space but if there is room in your engine bay I can't see why not especially if you can make something up cheaply rather than buying a kit.
If you haven't already guessed I am thinking of a cold air intake for by clubby
besides it is the closest a NA engine gets to forced induction
Any gains made should be at high car speeds so that would mean better cruising and hopefully better fuel economy at high speeds with a four speed gearbox that can't be bad
Mind you this is my thoughts reality tends to be different
OK, now to show my ignorance what is the reason you would need a plenum ? is this to try to regulate air pressure? or is it because you are generally using a ramflow type filter and it needs to draw air from all around the filter? what if I was to move the filter away from the back there and have air ducted straight into my weber?