In some cases, ROI is an easy equation. Branding and design, however, can be a lot more difficult to measure.
Instead of trying to isolate just branding or just design, you need to look at those things as part of the big picture.
Marks and Maker said, "There are SO many variables in business—from internal processes and operations, to marketing, employee problems, personal things, etc."
Set overall visibility and marketing goals and then try to evaluate where branding and design fit in.