Measuring leadership outcomes is tricky. What constitutes leaderships? What does a leader look like? I would think that leadership outcomes should equate to tangible outcomes that can be seen and easily understood. It would take more than survey data to convince me that an organization was developing leaders.
Like revenue streams, your outcome data should if possible come from multiple sources. We use survey data and journal data to measure outcomes. The survey data provides the foundation of the outcome and the journals provide the detail. Funders expect the survey data but will give money for the data found in the journals.