Bearing in mind that the temperature they serve Guinness at means that it is impossible to actually taste the stuff (and I'm not referring to the Extra Frozen stuff) I doubt that you will be able to tell the difference.
A lot of traditional stouts WERE of a lower gravity than we tend to demand today - they were drunk by Working men often in heavy or hot and often dangerous conditions - they replaced body fluids and it would not have been safe for them to be "drunk" on the job. Strong ales were saved for other times and places.
Quack!
Geof the Duck.