I can tell this one will take up too much of my time in brand experimentation... I used a 9% abv Imperial Stout, Appleton SB, and Kamora (terrible, saccharine; Mr. Black is far superior than any other coffee liqueur I've tried, and I'm continually wasting money trying other brands)... The ingredients I employed resulted in a flavorsome but too-sweet swill... I did gas-off the stout (stirring). I can't imagine using Guinness due to its carbonation, but I'll do it for science.
I have had this with both a standard proof Jamaican rum (Appleton) and Irish Whiskey (Jameson) and they were both excellent.
Not sure about the ‘shake all ingredients’. First time with the Guiness was a bit fizzy! I now add it at the end and stir.
I've revisited the Sout Fellow and tried both stirred and shaken but I much prefer shaken. I used Guinness Original and it had the potential to be explosive in the shaker but OK if you hold firmly. I've added a note warning of this in the method. Many thanks for your feedback.