I haven’t heard that claim before. My understanding was that such a claim would be improbable or cherrypicking of some sort, as a priori risk-adjusted etc returns should be similar or identical but by deliberately narrowing your index, you do predictably lose the benefits of diversification. So all else equal (such as fees and accessibility of making the investment), you want the broadest possible index.
I haven’t heard that claim before. My understanding was that such a claim would be improbable or cherrypicking of some sort, as a priori risk-adjusted etc returns should be similar or identical but by deliberately narrowing your index, you do predictably lose the benefits of diversification. So all else equal (such as fees and accessibility of making the investment), you want the broadest possible index.