I tried a sample of my recent oatmeal stout which underattenuated at about 1.020, for a 3 gallon BIAB batch and a pound of oats, I couldn't really taste them. I used quick oats. Perhaps the underattenuation covers up the flavor/mouth feel you'd get from the oats?
I've never really noticed any flavor or mouthfeel from using oats. Maybe I'm doing it wrong or maybe my perceptions are screwed up. Or maybe everyone else in the world is fooling themselves!
This is why I want to try toasting them. From what I understand, that will bring out the flavor.
I've never done a side-by-side with an with out, but I do get a very chewy stout with oats. It's a big beer, though, so perhaps it would be just about the same without.