It's the heads that tend to crack. We had it happen once at a gliding club where I flew.
Airflow through a cowling is more to create high to low pressure around the cylinders and help cool air flow down between them, the case itself is fairly well insulated in comparison.
I reckon you're probably right in normal operations. However, it pays to be cautious and if you are mechanically sympathetic it just goes against the grain not to act to avoid any chance of shock cooling. Like leaning the mix ... do it right and it will extend engine life and pay dividends ... do it wrong and it "could" cost you dear. Finesse is everything!
IM