i don't think it would be good in a stout. stouts depend on low attenuation so that there is enough residual sugar to counterbalance the acrid/ashy flavors from the roasted malts. BS would attenuate too much, not leave behind enough (or any!) sugars, and you'll have an unpleasant beer. i supposed you could add back some sweetness via lactose, but then who knows what the right balance is, and lactose has its own flavor, etc. better off using a less attenuative belgian yeast if you want a belgian stout.
now if you want a dark or black saison, BS might be a valid choice... but look up black saison recipes. they don't have nearly as much roasted malt as an imp stout does. they also often use things like de-husked roasted malts to get color but not flavor (again, unlike a RIS).