I have always used Flaked Barley in my Imperial Stout. I think it helps give a chewy mouthfeel.
Classic Guinness recipe is 70% pale, 20% flaked, 10% roast
ive had an issue with this one for a while tbh.
i have made several dry stout iterations, and the most recent one i made is NOT a perfect clone, but IMHO is much closer than the near-smokey dry-barky simply BURNT taste of all roast barley, at least from the maltster who does the RB i use.
the beer i made was 7% black malt, 5% RB the rest golden promise with 5% sucrose in an attempt to get it drier (this actually failed since i accid. mashed way too high). note 0% flaked barley, which i would actually amend next time i try this
i dont drink guinness super often, and i mean then next is : what IS guinness now, and what was guinness at different times?
but from all the multiple sources of guinness products ive had i feel like that 10% RB, 20% flaked 70% pale is just a lazy "we solved this a long time ago so dont think about it" homebrewing thing. ive made beers with this grist and they never tasted like guinness, way too assertive, just off in a lot of ways.
next time around id probably do a 149 mash, 8% black, 4% RB (guinness's roast is so smooth and in the background). 3% flaked barley and the rest some pale malt. as well as trying to figure out the water profile