I always thought the main culprit of gas port erosion was the bullet shearing against it, not the gas from powder burnt. The type of powder used is probably a factor is GP erosion, but I believe the main cause is the actual bullet rubbing against it. Gas port erosion is natural, and will happen no matter what type of powder is used.
Faster powders do have their downsides too in regards to throat erosion. GP erosion from slow powder is news to me, but I wouldn't consider faster powder always better at all.
My line of thinking has always been, fast powder with light bullets, slower powder with heavier bullets as a very general guideline for ideal performance since that is what most manuals imply. ( many manuals only have data for slow powder with heavy bullets and vice versa )
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