So I have brewed this recipe 3 different ways. From my experimentation and tasting side by side with guiness from a can, I can tell you the following:
1) British Ale yeast will get you closer to the AA and flavor than either Irish Ale yeast or British Ale Yeast II.
2) In a side by side comparison, this recipe will exhibit an obvious bitterness over actual Guinness. I suggest a later hop addition or cutting the hops severely.
3) Mashing the Roasted Barley gives this recipe too much of a roasted flavor. I have studied up on this and apparently, Guinness is actually made by mashing the grain separately according to some rumors. This seems to make sense because when mashed with the grains it throws the PH way out of wack, and it also imparts an astringency that while interesting in character, is not present in actual Guinness.
4) If you are not trying to match Guinness very closly, I find this recipe superior in flavor, character, complexity, and aroma. You can safely ignore 1)-3). Bottle conditioned, I think Guinness from a can still has better mouth feel and head appearance / retention. I'm also not sure if I'm being cynical here but I feel like Guinness may be using xantham gum or some other heading agents get better head retention.