500J is a lot of energy. Ballpark estimate, the feather would hurt quite a bit too. It'd have a similar impact to a 1kg steel ball at 31.6m/s (assuming it's 1000x less dense than steel) in terms of energy dissipated per unit area of skin, which is something like the mass of 7 baseballs at the speed of a typical baseball in the major leagues. I'd expect significant blunt-force trauma from a broadside feather at that speed. I'd expect much more severe injuries if it struck with a different cross-section.