Win 11 had HDR automatically enabled by default so I just left it on. Yes Battlefield 1 has HDR option but I noticed a massive improvement in graphics detail, also in Elder Scrolls online with no loss of FPS, but I didn't notice any visual difference in newer games such as COD Cold War or Cyberpunk or Metro Exodus Enhanced. However in COD I am getting an extra 15-20 FPS, but I didn't see a difference in FPS in other games. I haven't had chance to run any benches yet will try tomorrow.
One thing is I found every game had stuttering, after some research I found out it is caused by Win 11's in-game recording (a bit like Nvidia shadowplay). There is no option to turn this off, so after disabling it with a registry hack, the stuttering had gone.
Initially I hated the win 11 interface (taskbar grouped icons) and the start menu to the extent I found it unusable (Microsoft trying to copy Apple's minimalism over functionality) but after a lot of registry tinkering and third party tweak programs I have got it looking like win 10, so after my inital annoyance with it and a lot of messing about I'm now quite pleased with it.
I'm looking forward to game developers implementing Direct Storage for faster load times.
If you do the upgrade option rather than a clean install it's quite easy to revert to Win 10 as the old OS is kept on your disk. Installation was really easy just one click of the install icon and it took just over 30 minutes. All my settings /desktop etc were retained with no loss of any data. I didn't have to reinstall any drivers or anything.