I certainly wouldn't entertain run flats on an E63.
As mentioned, the ride is already hard and with the stiffer sidewalls you'll not only find it uncomfortable. You may then realise that Mercedes rims aren't the strongest that you can get. They will crack and split if you're lucky, if not then pieces will be flying off them.
For what it's worth, I had Eagle F1s on the back of mine and rated them highly.