In my experience, 500lbs of energy is enough to bring down deer. More may be desired, but not necessary.
Here's a video of a pair taken at 400. Please ignore the camera work, this was 4 years ago, before we actually started to seriously film our hunts.
A tripod would have made a world of difference.
This was with a 243WSSM, but the bullet I was shooting, was being sent at about 200 fps SLOWER than a standard .243 would send it.
Use a good bullet, at the right speeds and there should be no issues taking big deer at 600 yds. (or more with the right system)