Crab,
Having played the Weeny Roast Scene as the Weeny....I can support exactly what you say.
We did not wear Long Johns as they were not even considered but in my case....everywhere there was double or triple Nomex (as in leg pockets for example) I receive fairly minor burns and where there was but a single layer the Burns were more serious. The worst of my burns were above the Knee where the single layer of the Flight Suit was stretched fairly tightly and on my hands after the Nomex/Leather Gloves burned away.
Keeping the Nomex clean and free of dirt, oil, and grease also improves its ability to protect.
My Problem was that I had to stay in the fire as we were airborne and someone had to mind the store.