Well, i activated it on my laptop (i7-9750H, GTX 1660TI) and the fans are sensibly quieter when both ED instances are alt-tabbed (my steam and my epic accounts)
But last Saturday i encountered something weird that might be related to this settings:
I had my epic account parked on my steam account carrier. My steam account ship was aIso parked on the carrier. I plotted a carrier jump then i did some chores around the house. The Steam ED was active on my screen during this intermezzo.
Heard the regular carrier jump noises so i came back, alt-tabbed to my Epic account, launched from the carrier and... i noticed that i'm still in the previous system. I assumed it was a refresh glitch or something and instead of docking to the carrier i did a low wake to scan the system.
Bad Idea - i was still in the previous system, but after the supercruise transition the carrier was nowhere to be seen - the glitch cleared, the system refresh completed and my steam carrier was no longer listed to that system. While i was still there...
TL: DR - the carrier jumped, on my steam account i could see than i was in the destination system, on the epic account the jump never happened, i was still in the initial system, but when i transitioned to supercruise (low wake) the system refresh completed and the carrier disappeared.
Now, it could have been a FDev glitch - some sync issues between Steam and Epic (they did had some issues in the past like multicrew failing etc)
Or it could be related to the Background FPS limit