Hehe, I think it's a good idea but it will be a very dark stout!! The taste will be really intense!! I have a recipe, an Imperial Russian Stout with 1kg (it's 20% of my grain bill) of chocolate malt in 20L and I get a very, very dark beer, thick and with a lot of coffee and chocolate tastes. In my second version, I reduced to 0.8kg and the taste was the same, no difference. It's good if you like stout (it's not a Guinness!!). Good luck!