My X had the battery replaced by Apple late October. My capacity is 100% and it runs iOS 15.4 extremely well. When compared to 14.8.1 there is no discernible difference I can gauge but every device is different so if by some chance it doesn't run well on your device, you won't be able to return to 14.8.1. I upgraded using iTunes and the full 5.5gb update so I can't compare it to OTA upgrade