The .257 Wby is a bit of a temptress -- lazer flat trajectory, but with a light bullet of relatively small diameter. I have one, enjoy it, and have used it successfully on some longer shots. I've also owned a .257 Roberts for nearly 30 years and have shot a good number of deer with it as well. So I'll put myself down as understanding a fair bit about what a .25 cal bullet in 100 - 115 grain weights is going to do and what it's not going to do.
At 500 yards, with most deer-class bullets you're going to be down around 1100-1300 ft-lbs of energy. No, energy doesn't kill deer. But it's a good indicator that there isn't going to be a lot of hydrostatic shock going on when the deer is hit. Nor are you likely to see an exit wound.
Now, what you will see with a good, precise hit is a dead deer. But more likely than not, it's going to run a bit. Not a big deal in the middle of a snow-covered field where blood sign is impossible to miss. But if that deer is way down yonder cutline in the middle of a lot of green growth, and you forgot to get a very clear reference on a nearby landmark before pulling the trigger...
I guess it just depends on how good you are as a marksman, and how favourable the conditions (both shooting AND tracking) are. Myself, as I've become more comfortable with longer shots, I've learned to assume that as range increases the need for bullet diameter and full penetration also increases -- because there won't be much shock and a good blood trail is often a great boon in recovering the animal.
Myself, I don't really think of my .257 Wby as an ideal 500 yard deer gun. It will do the job, sure, if I do mine. But I don't think it's the best tool for the job.
As for what is the best tool, I think that's the pair of boots that would allow me to cut that distance in half
