Ahhh, shock cooling, that old chestnut
Say again does an engine cool more rapidly after shutdown? I wouldn't have thought so. The issue with shock cooling (if indeed it exists) is the sudden change in temperature between the cylinders and the block.
After taxiing in and running the engine for a few minutes at 1,200 rpm or so to clear the plugs and equalise the engine temps we turn the engine off. The cowling acts as an insulator and the cylinder heads and the block cool down at more or less the same rate. Compare this with a paradropper (a bad one):
Max power climb to drop height, takes 10 minutes say. Engine at the higher end of the normal temp range. Cylnders nice and hot, oil nice and hot. Get to the top of the climb. Reduce power, loonies jump out, stuff nose down to go and get the next lot. Now you have cold air being forced over hot cylinders due to the "efficient" ducting in the nose. Cylinders are producing minimum power and therefore minimum heat. They cool rapidly. Meanwhile the engine block is out of the breeze coming through the nose and is also being kept warm by the engine oil that was heated on the way up. The temperature differential between the heads (cold) and the engine block end of the cylinder barrel (hot) will cause contraction on one end and (relative) expansion on the other. Repeat this cycle on a busy dropping (or glider tugging) weekend and see how long your cylinders last. It's no coincidence that a lot of para clubs opt for turbine power when they can afford it (better ROC too).
Normal operations shouldn't cause major problems it's the extremes mentioned above that keep the cylinder manufacturers in business!