Just for sake of discussion, do you have 1-5/8 headers and are simply wondering? Or contemplating those or bigger and trying to determine how much you might give up if going with 1-5/8? I know some might spec their cams with a touch more exhaust duration at the least to try and possibly minimize the difference if they have to use a smaller diameter than what might be optimum. That’s in the build stage of course, sometimes you gotta work with what you have, or can fit, afford or find.