Well, the old staging method tried in the last launch was to rotate the entire vehicle stack and "fling" the ship off. That obviously has a performance penalty

Well, the old staging method tried in the last launch was to rotate the entire vehicle stack and "fling" the ship off. That obviously has a performance penalty in that you're not going to be pointing in the correct direction when you come off. I don't know if the 10% was compared to that or compared to a hypothetical "straight" staging with pushers, though.