The proposed percentage of roast barley isn't out of line with recipes I use. I wouldn't worry too much about the roast barley's color rating either. While that color is high, it doesn't necessarily mean that its more acrid or harsh. In talking with the team at Briess, I learned that the acidity of roast grains will actually DECREASE as the color gets very high. By 600L, the grain is nearly all carbonized (think charcoal) and it is no longer a significant acid producer. Since there is likely to be unevenly kilned grains in that product that are more carbonized, its more likely that it has similar acidity as other roast grains. If you look at the results of BrauKaiser's roast acidity testing, you'll see that most roast grains have somewhat similar acidity, regardless of their color rating.
The most important thing for brewing a good stout is to have sufficient alkalinity in the mashing water to prevent the wort pH from dropping too low. If your tap water doesn't have much alkalinity, then adding alkalinity to the mashing water via baking soda or pickling lime is highly recommended. You can figure out proper dosing using something like the Bru'n Water software.