I brew an oatmeal brown ale with about 15-20% flaked oats in the grist. The last time I brewed it I didn't use the oats, and I didn't really notice a difference in flavor or mouthfeel - although it had been a couple of years since I last brewed it with oatmeal so take my recollection FWIW. The main reason I included it originally was for mouthfeel purposes, but after brewing the recipe several times over the years I don't really think it adds much body, if any.
I occasionally add some vanilla to this recipe, and I will probably try the toasted oat thing next time I do this, since I think the oatmeal cookie flavor would be pretty nice in there.