Originally Posted by JJayB

With only air cooling the heads go though a heat cycle, more boost more heat. After a time the metal starts to fatigue and get soft and looses its sealing ability.
A rockwell test will confirm the hardness of the heads.
FWIW, the factory race cars in the mid 90's would discard the heads after 40 hours due to sealing issues.
PCA isn't the only place to run a turbo, NASA and SCCA offer some nice competition.