I first used acid malt last year in a 10 gallon stout, but only used about 1.5% per the suggestion of the proprietor of the LHBS. (He said a little goes a long way). Couldn't detect any flavor contribution or sourness at that level, but the stout was pretty tasty ... a bit too sweet for my taste, however, and didn't have the Guinness twang.
Planning to brew another 10 gallon batch of Irish dry stout in a few weeks using the 3% acid malt method (0.44 lb. for 10 gallons). Local water is pH 8.5 and very soft. I used approx. 3 TB gypsum to the mash/sparge water last time. This time my salt additions will be more exacting, using a combination of gypsum, chalk, epsom salt, Calcium Chloride and baking soda. I may also add the roasted barley late in the mash (or mash it separately @ 160F) depending on the initial pH reading.
My understanding from reading Michael Lewis' book on Stout is that the Dublin Guinness brewery adds the roasted barley/sour extraction to the boil, as told to him by Guinness executives (this is also stated on the website). He states that they add it to the mash at the London Guinness brewery, and elsewhere around the globe. Methinks there is more than one way to skin this cat.* Cheers.
*No cats will actually be skinned in the making of this beer.