I'm not the OP and I don't have any experience with the ported Shield myself, but last year Julie Golob of the S&W Shooting Team used one at the IDPA BUG Nationals where there were a few low-light stages.

She was using full-power defensive ammo and she claims that the flash was not too distracting.

Watching the video it didn't seem like the flash was much more than a non-ported short barrel pistol, but different ammo might have different results, of course.