The authors conclude that the power of variables other beta to explain average returns invalidates most CAPM applications. They Specifically reject using the CAPM to estimate the cost of equity capital and to evaluate performance of mutual fund managers.
This approach with corporate taxes does acknowledge tax savings and thus infers that a change in debt equity ratio has an effect on WACC (weighted Average Cost of Capital). This means higher the debt, lower is the WACC. This Modigilani and Miller the modern approaches of Capital Structure Theory