i was actually curious of the RS content in popcorn so i looked it up. i figured it was probably not very high since it's heated. there are .3 grams of RS in 100 grams of popcorn. i guess if you are eating a ton it will add up.
I think this also depends on the cooking method. If the hull isn't properly sprouted, the RS content goes up, though I definitely feel some discernible benefit after eating cooked popcorn.
One can also simply eat the hulls for significant RS, but have to be careful with teeth