Mickey,
I've never seen a .260 Remington at Hawks Ridge 1,000 yard matches, except when it was being developed & a famous shooter from Tennessee used it at the Nationals. He came in a few places up from dead last.
The .284 case is about the size of a .30/06. There is a long history of both the 6.5/284 and the 6.5/06 (improved) performing well at 1,000 yards, with bullets in the 139-155 grain region. To the best of my knowledge, not much for the .260 in Benchrest.
Which isn't to say it won't work, just that it isn't "benchrest tested." For ranges shorter than 1,000 yards, and with lighter bullets, it is probably just fine.
Another option with factory brass would be the 6.5x57, Can't remember if Lapua makes brass for this, and RWS brass is getting hard to get. But it is a nice case, based on the 8mm, not the 7mm Mauser (i.e., has a shorter neck, longer body).
If you're really going to shoot 1,000 yards, want something proven, and have to choose between the 6.5/284 & .260 Remington, I'd go with the 6.5/284. If you're going to shoot no more than 800 yards with bullets in the 125-grain region, the 260 is likely a nice chambering.
FWIW