Does holding Thai PR long-term (say 15 years) make it easier to gain Thai citizenship? I am a western man.

Thank you.

Only a little bit, in that you will get some extra points. But having PR usually means that you would qualify for Thai nationality, as many requirements are more or less the same.

Mario is right. You get a few more points for having PR for 10 years but you probably have well over the 50 points our of 100 required anyway and you start losing age points after 50. You get no more credit for having 90+ points rather than scraping through with 51 because it's only pass or fail. In the covering letter to the ministry Special Branch just confirms you got more than 50 points without disclosing your exact score.

So there is effectively no advantage in having PR for more than the minimum 5 years before applying for citizenship. I had PR for 14 years before I applied and waited in the queue just like everyone else without any special privileges. Waiting too long might actually disqualify some applicants, if they retire or otherwise stop working before the end of the process, since you must maintain a valid WP right up to the bitter end.