It is not necessary to sour the beer to create a dry stout. Guinness' method probably does not include a 'souring' step. Their naturally low alkalinity water produces a more acidic extract when they perform their separate roast grain steeping step.
Use the Guinness method to brew a dry stout. Use RO or distilled water for its low alkalinity. Mash all the pale malt and barley without the roast grains. Steep the roast grains separately in RO or distilled water and add that 'flavor extract' to the wort from the pale malt mash. The pale malt mash pH will be in the proper 5.4 range with minor additions of calcium chloride and gypsum. The pH of the flavor extract will be well below 5.4 and it will help bring the overall pH of the finished wort down and produce that distinctive acidic perception that the dry stout style exhibits.
I was planning on cold steeping the dark grains, per Gordon Strong's book, and you believe this will create the same effect?
That would be nice because I was not looking forward to the souring process.