Reflective walls will equate to more photons reaching the leaves.
The real question is will the effort result in an ROI?
With HPS very likely.
With directional LEDs as much. It would greatly depend on the distance of the fixture to the canopy.
If the cost and effort is low, then why not slap on a coat of paint?
One could make a case that white paint is more effective than mylar due to the differences in the type of reflections.
I think the best case was made by
@ANC when he said his plants lean toward the white pained wall.