Mileage gains, if any, are secondary. EGR reduces combustion chamber temps to keep NOx levels lower. GM must have seen a need for more EGR under higher load on the L31 so that's when it's active.

Be careful not to strip protective coatings / platings from components during cleaning process. Acids and bases are both very capable of doing that. I wonder if the "electrolytic rust removal" process would be effective here. It does a great job removing paint on parts that aren't rusty. I wonder how it will work on carbon?