The shear strength of the threads is far less than the 20,000+ pounds of force placed upon them during the combustion event. Were it not for the interference fit--which remains quite adequate at modest CHTs--the head-barrel fit interface would fatigue and separate. When the CHTs get excessively high the interference fit is lessened. This can result in failure since the threads alone cannot sustain the load.
For this reason, the OEMs recommend controlling CHTs to around 380-400dF as a matter of routine. It's good advice.
There is a combination of proper interference fit during manufacture and controlled CHT during operation which results in success. For this reason, if the need is high power, it can better be accomplished LOP where CHTs are lower, pressures in the combustion chamber are lower and the metal is stronger.
All cylinders are not created equal where the interference fits concerned.