I did 16 lbs for a 10 gal batch. My stout ended up being right at 11%. I think keeping the darker grains lower is a good idea. I did two different batches, the second with less intense grains, and it came out better. The sweet potatoes put in a great flavor, but I think if I do it again I will do more potato. I liked the effect that baking the potatoes had; I baked them until some were caramelizing on the edges and it gave a nice roasted/sweet flavor. It was nice to get that flavor from the potatoes instead of the malts. I doubt the potato flavor would be too strong with 10 lbs because sweet potatoes aren't near as starchy as white potatoes.
Hope that helps, and good luck!