Rhian (Bigngreen) has a lot of experience elk hunting, more than most of the rest us and he knows what he's doing with his 270 which is a lot like a 7 mag which is what I used to hunt everything with. After I cooked the barrel in the 7mm I got a 300 WSM
for all around Montana hunting with special thought to elk. When it comes to elk hunting, I'll grab a 300 mag (currently a RUM) or better for that little extra insurance factor and they are going to do a better overall job of putting it down quick.
That said, my #2 LR rifle will be a 6.5 WSM and I would not hesitate to use it for elk if my 300 were to become unusable for some reason. I would probably not shoot to 1K on a bull but wouldn't mind stretching it out on a cow if the opportunity came up. Matrix also has a good 160 gr 6.5 bullet. With a 300, I would take a longer front quartering shot on a bull if that's all I had. I wouldn't take that shot with a 6.5 WSM unless fairly close and with a larger, higher SD, bullet like a 140 minimum.
I didn't know Bigngreen had a video of an 800 yd kill. I would love to see it. Gotta link?