McKinsey’s report from 2018 is looking into the business value of design. They are comparing companies that did their McKinsey Design Index assessment, which is their design maturity tool. If you want an argument for how business is affected by a robust design environment, this might be a good read for you. As an extra, Invision did their own summary of the 2018 McKinsey report.
The Design Maturity tool from the Artefactgroup was my tool of choice back then in 2016, to conduct the design maturity survey at airberlin. It might be the oldest of the tools but is still a valid one.