According to the CISM Review Manual, the most important metric to present to senior management when reporting on the performance of a risk mitigation initiative is the cost and associated risk reduction, as it demonstrates the value and effectiveness of the initiative in terms of reducing the likelihood and impact of the risk. The other metrics may be useful for comparison or analysis, but they do not directly measure the performance of the initiative.
References = CISM Review Manual, 27th Edition, Chapter 4, Section 4.3.2, page 2091.