I found potato starch to be the best "out of the bag" RS supplement. I had to switch over to plantain flour in order to cut nightshades from my diet, and a lot of the benefits went away. But now that I've started including high RS parboiled rice in my daily diet, all the bennies are back! So I'd say stick with potato starch or parboiled rice. I'm only just beginning to work legumes back into my diet, so I can't really comment on those yet as an RS source.
I eat cold potatoes a few times a week post workout and have noticed relief from issues that began popping up as I was low carb. My reflux is gone, indigestion is gone, bloating is gone. Probably has something to do with my body being happy higher carb, but I think cold potatoes and cold sweet potatoes have helped with digestion.
I find that I eat enough vegetables and fruits that the morning tablespoon of potato starch in my smoothie makes no difference. I am half way through the second bag, but when I finish it I will not buy a new one.
I know some folks like to just take straight up potato starch, but I find, that if I am going to consume something I would like it to resemble an actual food. So, I eat lots of potatoes and sweet potatoes and will let them cool before consuming. Also unripened bananas. I don't see the point in over thinking it.. though I know a lot of us in this "community" do just that.