What are you shooting with the 500 mm lens, that would benefit from a slower shutter speed? If it's birds or other wildlife, they will need to be perfectly still to get the shot. I can see where the IBIS is a game changer for people that want to use the camera for walking around, especially with that new 100-500 RF zoom. But when you are attaching it to a big lens like the 500 f/4, you're probably using a tripod so the lens IS should be more than enough to deal with the regular vibrations.