Randy Mosher provides a black pepper porter recipe in Radical Brewing:
6 lb Munich
3.5 lb 2-row
1.5 lb C120
0.75 lb chocolate
0.4 lb black patent
1.5 oz Target (9%) 30 min
.75 oz Perle (8%) 30 min
.75 oz Crystal (3.5%) 5 min
0.4 oz freshly ground black pepper (end of boil)
0.4 oz freshly ground black pepper (in chilled wort)
Any british ale strain
His notes are important: "It was a well hopped beer, and the pepper was added to provide character. Notes of pepper could be picked up, but the pepper heat was masked by the high BUs, and the aroma by the hope fragrance. It would be difficult to over-hop this beer."
Now that I have spent the time typing the recipe, I would not use his porter base recipe at all. But the pepper volume should help. If you are adding roast barley to the stout, I would up it a just a little to balance the bitter roastiness. He also notes elsewhere in the book on black pepper, "adds depth and complexity to beer, especially dark ones. Enhances other flavors. Use in small quantities, less than a teaspoon per 5-gallon batch." I googled it, and 1 teaspoon is approx 2.1 grams of black pepper.
So, for a dry stout (Guiness style), I'd go with 1/4 teaspoon at the end of boil and 1/4 teaspoon (for heat) in the secondary (to develop aroma). Hope that helps.