Perhaps another cause of the paint coming off is going from -20degrees to 80degrees constantly in winter? I can't imagine paint would like that much.
The plenum is on the opposite side of the turbos so getting excessive heat under the engine bay is more likely to cause paint damage on the hot side.The logic is quite flawed.
The valve and timing covers are the same sort of alloy, one would expect them to lose more paint closer to the turbos.
If it's meant to indicate 'serious thrashing that leads to extreme heat beyond normal use' as you suggest. I have to say that I've never seen the paint come off the plenum at track days as an instant result of this.
The most obvious areas to look for 'over heating' are the oil and cooling systems or the things they bugger up when run to extreme temps. Like compression and cyl leakage.
You buy a car from japan this old and you take your chances. Given you can't walk up and test these motors, Plenum paint is the least of your worries and so this BS myth belongs in the bin as an old wives tale.