If the density wave speed closely matches the orbital speed of the stars, what happens to the visual arm structure?

The arms become more open and less intensely lit because the compression phase is shorter and less dramatic.

The speed at which the density wave propagates relative to the orbiting matter is a key determinant of the visual appearance of the spiral arm. When the wave's pattern speed nearly matches the orbital speed of the stars and gas clouds, the duration for which any given piece of material remains within the high-density, compressed zone is significantly reduced. If the compression phase is brief and less pronounced, the resulting star formation is less continuous, and the illumination provided by the massive blue stars is less sustained across that region. This leads to spiral arms that appear more open, less tightly wound, and generally less intensely luminous compared to galaxies where the pattern speed creates a long, slow compression event.