The 6.5/284 is without a doubt the hottest 1000 yd round going, but in my opinion it would not be your best choice for a beanfield rifle.
A 1000 yd round only has 2 primary qualities to be popular. Accuracy and the ability to stay super sonic at 1000 yds. Trajectory is not a major concern as the yardage is know and the shooter simply dials his scope up. If his scope doesn't have enough adjustment, he installs a 20 MOA scope base.
Flat trajectory is much more valuable to the hunter. In a perfect world, there would always be time to use a range finder and dial your scope up the proper value for a shot. That doesn't happen very often and you must estimate your range and use hold over to adjust for bullet drop.
There are alot of flat shooting rounds out there but I would recomend a 25-06 Ackley Improved. It will send a 100 gr. Nosler Ballist Tip out the barrel at 3700 FPS. This is the same as a .257 Weatherby, with nearly 10 grs. less powder (longer barrel life) and without having to buy expensive brass.
The 25-06 AI with a 100 NBT drops 20" less than a 6.5/284 with 140 gr. bullets at 500 yds. This gives you more margin of error when there's not alot of time.
As I said earlier, there are alot of flat shooting calibers out there. I used to shoot a 270 WSM and it was a fine beanfield rifle, but with nearly double the recoil of the AI, practice got to be work after awhile. Pratice is the first responsiblity of a hunter wanting to take longer range shots.
Do some research on the web and see what rounds are out there.