I'm no expert by any means. Just a few of my own experiences.
I've shot a few timed courses over the years that started outside of a building during midday(sunny)to the inside of a building with low light conditions. As well as courses simulating a building sweep with low light conditions and pop-up targets. Some targets BG, some GG.
Have used shotguns with GRS, bead and rifle sights. My best times were always with a florescent bead. Especially on the course going from daylight to low-light conditions. Some of the BG targets would pop up next to each other within a split second. I didn't loose much time on these targets with the GRS versus the bead or rifled sights. The targets I lost time on with the GRS were the BG targets that popped up within seconds of each other that were several feet apart. Seems the further the targets were apart when they popped up the more time I lost using the GRS compared to the other sights.
But in all fairness, I've shot more with bead and rifle sights over my lifetime then the GRS's. Maybe I just need more range time with a GSR.