Actually, I used to work with a Sam Adams Master Brewer, and he told me that ALL stout, by its very neature, has less alcohol than many other varieties of beer. There’s just so much stuff that goes in it, there’s not enough room per volume for much alcohol.
That “Sam Adams Master Brewer” knows far less than he thinks he does. The Imperial Stout (aka Imperial Russian Stout) is as strong as a barleywine.
http://bjcp.org/styleguide12.html
American premium lager (Sam Adams): 3.5% to 5.1% alcohol by volume.
Imperial stout: 8% to 12+% alcohol by volume.
What differentiates Guinness Extra Stout from Guinness Stout?
Sour stout.
The extra stout has some soured stout blended into it. This comes from the Guinness company. It’s an intentional variation in the flavor. I drink both the draught and the extra.