I can't speak as for how it will affect judging for being out of SRM for the style.
However, I use 8 oz. roasted barley in my hazelnut stout and it is delicious. It gives it a little espresso/coffee flavor.
My SRM calculates to 171.
My only suggested change would be upping the IBU. I use a rule of thumb of IBU/OG of 0.7 to 0.6 for sweet stouts. The lower the number, the sweeter. The higher, the more bitter. Yours is 26/54 =0.48.
Of course, I don't know how that change would put you further out of the style guidelines.
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