Shock cooling has been done to death in every pilot forum, airport bar, aviation magazine, Usenet, engine manufacturer's literature...
And nobody is any wiser.
This is because nobody has put a load of engines on test rigs, shock cooled them repeatedly, and then checked them for cylinder cracks. Then repeated the experiment for different engines, different CHTs, different cylinders, etc etc etc
The best data I have seen to date is an article written by somebody who was dropping paras or pulling gilders or something like that, who used to get through a lot of cylinders, and then changed procedures to avoid reducing power until the CHT was below a certain figure. I have the article.
And this is probably the answer. If your CHT is 500F (which is itself way OTT) and then you cool by 100F, that is far worse structurally than the same temperature drop if you are starting with a CHT of 300F.
And on a typical landing approach your CHTs are low by the time you are shutting off.
The trap is on an IFR arrival when they have held you fast and high and then force you into a -2000fpm descent. It can still be managed if you know what you are doing, and think ahead a bit.