Everyone has a personal tolerance for risk. If you are diversified, then you decrease the risks that you are exposed to - possibly below your personal risk tolerance. That allows you to use leverage to increase your risk back up to your tolerance, which also increased your expected return.
So yes, diversification (plus leverage) absolutely increases expected value, if you hold risk a constant.
So yes, diversification (plus leverage) absolutely increases expected value, if you hold risk a constant.