If I were doing this, I'd mash at a low temp (maybe 147?) to get a more fermentable wort, so the FG would be closer to the normal dry stout range, but with more alcohol. And Klicki Jim's right about increasing the bitterness to match the OG increase. Although, 1.5lbs of roasted barley will provide some roasty bitterness on its own.
As it is though, it should be a good beer, just maybe not as dry as you'd expect.