Sorry, I guess I wasn't clear. When I calculate my strike water at say 1.3 qts/lb, for 10 lbs that's 13 quarts. Then I add the dead space water, 2 qts in your example, for a starting water of 15 qts. This (15 qts) is the number I use to calculate the strike water temp, but 13 qts is what I use to calculate mash thickness.
I agree though, I don't think you would really notice it in your beer. As long as it is thin enough to hydrate the malt, the temp will have a larger effect than the thickness in my experience.